No that was definitely your question. I know you, Becuase then you go on, that it makes sense for blood elves to join the Horde. If the King joins the Blood elves then makes sense that the King would also join the Horde. But Anasterian Sunstrider would never join them. Also he was forced to join the Lordaeron Alliance back in Warcraft 2 thanks to Alleria. Even back when the High elves first met the Barbarian Humans, Anasterian father. That be Kael’thas grandfather never intended to work with humans in the Trolls wars. Only use the Barbarian humans as a tool, a weapon against the Amani Troll. However not all High elves agreed with the King. Some stayed with the Humans, building Dalaran. The same Anasterian, when he ordered all High elves to come home after seconded war. Some did not listen and stayed with the Alliance and is still there. Fighting for the Alliance.
Blood elves was only playable to the Horde out of necessity and to give Horde a pretty race to balance the player population, not for it to make sense. However if the Blood elves had stayed as they were back in begning of TBC…then it would had made sense and given them a connection to the Horde as they were back then, seeking darker powers. Becoming more and more like Dark elves.
True but also Thrall did not “care” for magic so he did not see the benefit of Blood elves joining the Horde. Lor’themar had to “bribe” Thrall to join the Horde with knowledge (reports) of seeing uncorrupted orcs in Outland. Hence why Horde goes to Outland in the first place. To seek out these uncorrupted orcs.