Large dark gray object hovering in the sky

While driving to work at 8:15 this morning, I noticed an unusual shape in the sky.  Because there were a lot of clouds, I initially thought it was a unique, very dark gray cloud formation. 
When I was stopped at a red light, I could get a better look at it, and it was clear that it was not a cloud.  It was an object hovering in the sky.  I thought at first it might be an oddly shaped blimp, but there are frequently blimps in this area and I’d never seen one with this shape.  This was much more flattened and wider than a blimp.  I could make out what I thought were darker demarcations of some sort on it.  It was mostly hovering, but could have been moving toward the west very, very slowly.  
When I got to work, I found that 4 other people had seen it also.  At a little after 10:00 a.m., someone came in talking about it because they had just been looking at it still.  When I went out to look again at 10:15 it was no longer there.
((ADDENDUM FROM WITNESS))
Hi- This morning I reported what I thought was a UFO sighting - I have since found out that it was some unusual blimp flying over a convention for Blizzcon -  put on by Blizzard video game company.
Just wanted to clarify/update my report.
((END ADDENDUM)).
We have encouraged the witness to contact the local FAA Flight Service Station, to inquire whether a blimp was in the area, and perhaps in touch with the local airport tower or ATC.  PD