The biggest lore reason why the Blood Elves would not rejoin the Alliance has always centered around the actions of Garithos and those under his command. His sins have been forcibly passed down to the current Alliance from the Blood Elf perspective but I have three rather important questions:
Garithos was a Grand Marshal of Lordaeron, which is the current capital of the Forsaken and no longer within Alliance control, wouldn't that make the Lordaeron-"descended" Forsaken former members of Garithos' people?
If the Scourge had taken Garithos before his encounter with Sylvanas, wouldn't he have been Forsaken and thus a member of the Horde?
If the above questions would both be true, what justification is there beyond carrying on the same organization's name for the current Alliance to inherit Lordaeron's guilt?