Blood mages are an interesting idea but they would have to be horde only, and only if Sylvanis remains in control and keeps leading the horde toward evil.
You have to think about the “Evil” classes we have now and how they are seen lorewise.
Warlocks are mostly ignored, they are tolerated because their knowledge of demons is useful in fighting demons but are pretty much don’t ask, don’t tell status.
Deathknights are only tolerated because of their help in defeating Arthas and because Tirion Fordring personally vouched for them.
Demon hunters where only released because the world was being invaded by demons.
I’d be interested to see a blood mage class but there is absolutely no way to fubar the lore enough to make them acceptable to the alliance. The horde could have them if they want to keep portraying the horde as the “evil” faction but they would have to give the alliance a mechanically identical, but thematically different class, tide casters for example.
I really dont see them doing that. The current dev team is way too lazy to come up with 2 different ability name and animation sets for one new class.