i'm not sure about the difference between 7th and 8th but from what i've read, 7th is inferior to 6th because the former doesn't provide case citations for the terms. so, the newest edition isn't always the best.
If you're looking to a law dictionary for case citations, you're REALLY missing the point.
Black's is the place to go to get a quick sense of the meaning of a term. That's it. If more research is required, then you should go look it up.
Black's 6th was a jumbled mess of citations, random CJS references, and other detritus tossed in by West over the years. Black's had been sorely neglected for decades until Bryan Garner and crew tried to clean it up and make the definitions coherent and therefore actually useful.
The differences between the 7th and 8th are minimal, regardless of what West's marketing department tells you. The key is to buy the 7th forward. All previous editions are, for the modern practitioner, garbage. Legal historians may find some benefit from poring over the first or second edition, but that's it.