There are some real experts on this site, but the only thing that is underneath the slide is the disconnector. This trigger disconnector is just forward of the breech face when the slide is locked back on my M1911's. Is the slide blocked by that?
On a series 80, the firing pin block will drop down and prevent forward movement, but you are claiming this is a series 70.
If your Colt has one of those split ring bushings, maybe one of the fingers broke off and is jammed between the barrel and slide. I removed a split ring bushing and replaced with a solid bushing when the finger broke off.
I can see that removing the barrel bushing will be tricky if the slide is locked back, but removing that spring tension is important for diagnosis.