Isn't that essentially the same thing as the, much aligned by some, kill on sight lists? It would basically be promoting witch hunting and a digital version of ex-communicating people potentially based on nothing more than rumour and hearsay or because that one person has someone on their block list for whatever personal reason unique to them.
Say for example one player gets blown up by another player once for whatever reason. The player that got blown up gets a bit vindictive and decides to make sure to get as many people as possible to block that other player so that their Open play experience is an empty one. You know, just out of spite. That would sound like greifing to me via use of the block feature no less.
Personally I only block people I've seen in game, the forum or you-tube doing things I consider blockable, I wouldn't take anyone elses list as seriously for exactly that reason cheaters gonna try to cheat.
KOS threats don't really work on anyone who will happily switch modes, report players and use the block function. Which is another positive.