Philadelphia’s skyline is slowly becoming a blur of blue glass. The trend started with Brandywine’s trio of skyscrapers on the Schuylkill waterfront in the early 2000s, and now almost every new high-rise is being shrink-wrapped in yards of sky blue. PMC’s Schuylkill River tower, rising at the foot of Arch Street, will turn a blue face to the city. Same for the Toll Bros. condo project on Jewelers Row, the nearly finished W Hotel on Chestnut Street, and the growing cluster of lab buildings at the Science Center.

Philadelphia isn’t the only big city experiencing a blue period. Skylines from Shanghai to Charlotte, N.C., are filling up with similarly hued glass towers, pinched and prodded into a variety of shapes. Because glass panels snap easily into place and can be installed while a tower’s frame is still under construction, the material is popular with developers, who are under increasing pressure to satisfy investors and meet tight construction schedules. Blue is now the official color of global capitalism.

Don’t get me wrong: I have nothing against glass as a building material. I still have a soft spot for the sharply angled Cira and FMC towers: They’re dramatic shape-shifters that seem to alter their appearance every time you turn the corner. As the light changes, they play hide-and-seek with the clouds. But you can use that trick only so many times before it becomes a cliché. Our glass architecture is rapidly being watered-down right now. And, if that isn’t bad enough, those slick, scale-less buildings are also killing off bird populations.

So when I saw that glass towers dominate the master plan for the parking lots that surround the chocolate-brown Catholic cathedral on Logan Square, alarm bells went off. Exeter Property Group, the international developer that purchased the land from the Archdiocese of Philadelphia, wants to construct two skyscrapers on the block: a modest 245-foot-tall tower directly behind the columned basilica and an immense 685-foot companion on its north side, next to the Vine Street Expressway.

It’s one thing to turn the Schuylkill waterfront into a lineup of sculpted glass vessels. It’s quite another to place two reflective towers next to a major cultural landmark. As a former industrial corridor, the waterfront is largely a blank slate. But the Cathedral Basilica of SS. Peter and Paul sits in the thick of Center City, in the heart of the Parkway, in a dialogue with the city’s most esteemed civic buildings. That postcard-pretty setting is one of the views that defines Philadelphia.

To be clear, the height of the towers is not the issue here. It’s been at least 40 years since the Archdiocese erected a bland, 200-foot office building just behind the Renaissance-style cathedral. Another, equally bland high-rise, the Franklin Plaza Sheraton, went up around the same time, also in the backdrop. It is heartening to see the remaining parking lots that surround the cathedral finally be put into productive use. The problem is that Exeter is letting us see the design for only one of its towers — the smaller one, at that — even though they will work as an ensemble. How can we evaluate half a duo?

Remarkably, the city’s Civic Design Review board wasn’t bothered by the lack of this crucial information when it considered the project earlier this month. After reviewing the design for the smaller tower, the board sent Exeter’s architects, SCB of Chicago, merrily on their way. The lone objection to Exeter’s proposal came from board member Elise Vider, who is also the chair of the Design Advocacy Group, and who shares my concerns about glass towers. “Having a large reflective building behind the dome is troubling,” she told me. Because the new tower will face west, it will become a mirror for the afternoon sun.

But it’s not too late for Philadelphia to demand more scrutiny of this significant project. Exeter’s proposal is expected to go before the Art Commission at its meeting Wednesday. In contrast to the Design Review board, the Art Commission is immensely powerful. Because it was created to protect the integrity of the Parkway (among other things), it has the final say over what happens on that landscaped boulevard. At the very least, the commission should demand more information from Exeter about the second tower.

It hardly needs to be said, but the cathedral isn’t just any old church building. It’s the locus of Catholic life in a very Catholic city. While it may not be the oldest Catholic cathedral America, its construction was a symbolic retort to the nativist, anti-Catholic violence that swept Philadelphia in the mid-19th century. Architectural plans were drawn up in 1844, a few months after the worst riots. Many significant figures were involved in the design, but the most important were architects Napoleon LeBrun and John Notman, who gave the cathedral its Italian Renaissance character and large copper dome.

Architect SCB is clearly aware of the importance of the site. It took pains in its master plan to provide breathing room between the towers and the cathedral. The plan calls for 50-foot-wide, landscaped walkways that can serve as public plazas. It also stepped down the larger tower so its 18th Street portion would not loom over the basilica.

Despite that deference, there is reason to be concerned that the new skyscrapers could still overwhelm the cathedral and diminish its prominence in Philadelphia’s geography. To get a sense of what is in store, take a look at the Arch Street Presbyterian Church, the domed building squeezed between the two Comcast towers. The cathedral is a far more important building in the life of the city, but it will be similarly encircled. That’s why getting the towers’ architecture right is so crucial.

The good news, I suppose, is that SCB’s tower is better than the dull, blue monoliths that PMC and Toll are building right now. Clara Wineberg, the architect overseeing the cathedral project, told me that her team intentionally tried to keep the design simple to avoid competing with the cathedral. The model was Boston’s Hancock Tower, a glass skyscraper that hovers over H.H. Richardson’s famous Trinity Church.

There’s a big difference, however. The Hancock building sits at an oblique angle to the church, with a large plaza in between, while SCB’s tower will sit directly behind the cathedral. Its distinctive dome will be silhouetted against a glass backdrop. To keep the SCB tower from looking like a giant mirrored wall, the architects cut a deep cleft in the center of the facade. Wineberg calls it a “pleat.” There’s a second indentation on the Race Street side. All that adds character to what could be a generic slab.

Too bad the architects didn’t put the same effort into designing the base. In the renderings, the ground floor is wallpapered with brick panels. No doubt, the brick is meant to make a “contextual” connection with the brownstone Cathedral. That’s the same lazy strategy used by Toll Bros. on Jewelers Row. When did architects stop creating skyscrapers that were unified compositions from top to bottom?

Even though SCB’s home office is in Chicago, the city that invented the skyscraper, the firm is no stranger to Philadelphia. Its architects are responsible for two of the better, recent apartment towers in the city: the Murano and the St. James.

The firm is also designing the Laurel, a 600-foot, blue glass building that will overlook Rittenhouse Square. After extensive back-and-forth with the neighborhood group, SCB worked out a design that includes thick, stone-like bands between every floor, similar to those at the Murano. It’s not a perfect solution, but it gives the Laurel’s glass facade some texture and much-needed sense of scale.

There are other ways to keep glass towers from looking like generic slabs. The architects of Comcast’s Technology Center dialed down the slickness by corseting the exterior with muscular steel bracing. PMC, which recently hired the Gensler architecture firm to re-skin one of the old Franklin Plaza towers, broke out of the blue-glass rut by using smoky gray glass and thick steel bands around the windows.

Wineberg told me that Exeter has an ambitious construction schedule and wants to start work on the first tower this summer. Before that happens, Philadelphia deserves to know exactly what’s in store for this city icon.