You haven't significantly compromised strength by removing the last pin and the back of a drawer actually doesn't need to be that strong1 so I don't think it's a big concern.
But obviously it is best not to lose a part of a joint (even if only so the effort to make it isn't wasted!) so I'll list some options for you.
Sticking with the Blums
For future ...
I'd like to keep using this style of slides on the rest of my drawers. Are those slides just incompatible with dovetails, or is there some trick so that I don't end up with a missing pins?
I can think of a few solutions, but if you want to keep using dovetails for that rear joint, the simplest solution is surely to just change the spacing of the pins and ...
Screws or L-brackets to build a drawer unit?
Either method can be used to build simple pieces and, if done well, can yield pieces strong enough for service; long-term durability isn't necessarily guaranteed though, because of the friable nature of particleboard/chipboard.
My old ones hold themselves using wood f4 50mm screws, two on each joint. I'm ...