That might be internally or even thematically hypocritical, but people often are hypocrites and draw arbitrary lines in the sand. The Blood Elves embraced Fel magic out of sense of desperation despite their previous strong ties to the Light - this could be construed as a hypocritical response as well, but it was justified due to their desperation at the time. The Blood Elves, through their union with the Horde, permits the Ebon Blade to operate in their midst because of the strength the Ebon Blade provides them - a collection of Void-addled Elves running around and spreading madness and panic offers no advantages or strengths. The issue may well not be one of "the Void is worse than Necromancy" to the Blood Elves - for specifics we'll have to wait to see what happens in the Void Elf intro sequence and story.