Dark Rangers have nothing to do with blood elves, it’s a common misconception.
Dark Rangers were specifically a Forsaken thing and specifically under Sylvanas. That’s not to say the current forsaken couldn’t try to make more, but they are and have always been a Forsaken thing and it makes no sense to offer them as a blood elf customisation option because the dark rangers have never been affiliated with Quel,Thalas.
This is overlooking all the problems of restricting them to a specific class (Hunter) as well, because dark rangers are not a race, they are a very specific thing indeed which is (aside from Nathanos) the spirit of elven banshee rangers forced back into possessing their own corpses.
I cannot see the kingdom of Quel’thalas, especially after Sylvanas’ little stunt, supporting creating more. Lor’themar was explicitly clear to Sylvanas in the past: he will not tolerate his people being used as undead puppets.
This is enough for me to say that the blood elves do not view dark rangers with any particular favour, rather they probably look at them with disgust. So again: it makes no sense to have them be a blood elf customisation option no matter how cool it might look.
If, and it’s a big if, such a thing were added it would only be for a forsaken Hunter as a special option, but I cannot see blizzard allowing forsaken to have special treatment for a particular class without every other race demanding the same.
Some things just aren’t meant to be playable, and dark rangers are one of those things. The few that exist are the only ones, there appears to be no effort or method to create more given the vast majority of them were loyal to Sylvanas and thus are extremely untrustworthy.