I just wanted to ask for that feature as well and saw this feature request, I guess it's the same thing I want:

When editing two or more blocks that relate to each other it's sometimes impossible to construct geometries without seeing all blocks at the same time. So if editing a block would not hide the rest of the drawing but more or less "lock" everything outside the block it would simplify constructions a lot. Now I end up either exploding the blocks and then reassembling them together later (however with lines overlapping this is sometimes a tedious process) or alternatively I make a copy on a layer that I first include in a block, then delete afterwards (however that works only if nothing is rotated/scaled)

I know it's not that easy because a block can also be rotated or scaled when it is edited - would be very useful though. :)
it would be helpful to have the "outside" geometry still for snapping when editing inside a block...

thanks!
Martin