I mean.. thats the whole job for the manager? Through performance reviews and keeping track of the work their team is doing. If the senior is actually "unblocking projects" then it should be apparent.

Project not making progress => senior gets involved => project making progress. Or senior gets involved and some architecture decision gets changed to alleviate some risk.

If the senior is doing this but its a visibility problem, then it may suck but they also need to self-advocate if the manager isn't proactively keeping track of their team(like they are supposed to be getting paid to).