It means that Blink will be to WebKit what WebKit is to KHTML.
KHTML is still being maintained, and code is regularly ported from WebKit back into KHTML. I imagine WebKit will still press-on. Remember that Apple did huge things to WebKit to advance their own agenda, mostly in ways that enable WebKit to be used to create aesthetic and fluid user-experiences - whereas Google's main motivation seems to be performance. Considering Apple's solution to poor performance seems to be to throw more hardware at a problem I think this is where we'll start to see differences.