What's with the blue sky billboard in Old City?

High above the Benjamin Franklin Bridge, facing Second and Race Streets in Old City, a billboard defies its mission. It tries to blend in.

There's no text, no website, no hashtag. Just a sky-blue rectangle with wisps of clouds against the cerulean beyond.

The ironic, almost calming, view was named one of the top 10 places to Instagram in Philadelphia by the Democratic National Convention's local host committee last year.

So who's behind the billboard in the sky of the sky? An airline? A hot-air balloon company? A funeral home? Some sage commenting on our ad-saturated society?

PennDot came back pretty quickly with an answer: Keystone Outdoor, a company with about 110 billboards in the region. Keystone is more often in the news for battling with residents over sky-high digital billboards.

Why's a billboard company trying to disappear? Owner Joe Felici said the point isn't a billboard as much as a windshield to protect the two other Ben Franklin billboards: A SugarHouse Casino ad facing inbound traffic and one for Boomerang Office Furniture facing those headed to New Jersey.

"If I didn't have that windshield, it'd be an open V and it could blow out the face panels – this ties it together," Felici said. "Structurally, there's nothing better than a triangle."

Even if he wanted to slap an ad on it, Felici said, city and state law limits billboards to two commercial-use faces.

"You could never put anything up there," he said. "Even like a Nike symbol." Lovers of the cloud billboard, rejoice!

Initially, the windshield/billboard was black vinyl, but it went sky blue five years ago at the recommendation of the president of the company, Dominick A. Cipollini, who thought it would be more pleasing to passersby.

It's so unobtrusive that it took me five years to see it, though I'm not alone.

On a recent sunny morning, most people I buttonholed were noticing it for the first time. Their takes ranged from introspection to confusion.

Jonathan Langley, 38, of West Philadelphia, passes it every day on his way to a house he looks after at Third and Vine. He'd never noticed the massive sign before. Once he took it in, his thoughts flowed:

"Being as though I've got this cross on today, I guess my mind would be thinking something spiritual. Just being at a place where you don't have any problems." His mom is battling Alzheimer's and she's been his biggest cheerleader.

"Down here there's so many different things going on, but if we were up there we wouldn't be thinking of that. Sometimes I have these dreams about flying and I think it's because when you're high above, all that stuff that's down here, it feels better, you know?"

Janet Conforti and her husband, Rob, of Long Beach Island, N.J., had a more transactional take.

"It doesn't look finished," Rob said.

"It makes you stop and slow down and look at the clouds a little bit, disconnect from everything," Janet said.

"Or it's not finished," Rob said.

Outside the Fireman's Hall museum on Second Street, Ed Chaney and J.T. Williams were staring at the billboard, which the firefighters said they pass twice a month. This was the first time they'd noticed it.

"It's a waste of money," Chaney said. "There's already a sky behind it."

Heavy-duty vinyl billboards like the blue sky one cost between $1,400 to $2,000 and are built to last.

"The cloud vinyl will probably stay there for the life of the billboard," Felici said. "Unless someone comes up with a better idea."

Anyone?