so i could quite literally kill some legion and ncr members and be blacklisted from both, right?
To me, the answer to that should be very obvious: Yes, they both equally frown upon having their darling members killed by you
As a general rule, I tend to regard violence and murder as
very serious offenses, in terms of roleplay and immersion. I never, ever hurt NCR members, for example, because I - personally - consider them "legitimate", and therefore citizens, and as such, any violence towards them a crime.
Legion I consider illegitimate, "terrorists and insurgents" if you will, so, to hell with them!
That is of course player preference, but by generally considering violence
very serious, as a ground rule, you can avoid some hefty reprecussions at least early on. For example, when I first meet Legion at Novac, I am humble and respectful, and allow them to walk away, I don't engage. Quests requiring violent engagement I deliberately save for later, for when I am ready to personally "declare war" on them.
(This reminds me of FO3, where in the alien space ship that raider chick reminds you not to kill her, when pretend-fighting to gain the attention of the alien guards. Goddamnit, I don't need to be reminded not to
murder my fucking allies! Such immersion-breaking crap...
)