I'm looking for blackbird. the marker says he spawned in a building with no entrances or exits. there are no markers on any doors.

the building is one of the buildings with raiders in between Ticonderoga and Greentech Genetics. It has a red and whit ewater tank on the roof

I'm looking for really any solution to the problem, even if it means skipping the quest.

Slight spoiler....

Are you sure there is no entrance at all? Use Local Map view in pip boy to spot any doors, hatches etc

nope nothing

I think I found some stuff in a desk (a letter or something like that) that was enough to send me back to Doctor Carrington.