The last thing you did in the Blood Elf start zone, well before Cataclysm, was to go to see Thrall as the representative of the Blood elves, recite the loyalty oath and join the horde.
The blood elf start zone is basically this.
Sunstrider Isle:
Learn how to absorb mana from creatures (tutorial for racial)
Cull predator animals and few ents that have got out of control reluctantly.
Learn and revere those that fought valiantly against the scourge invasion (minus Sylvanas at this time)
Deal with wretched threat.
Eversong.
Pick up a dead outrunners manifest and deliver it to the rightful recipient.
Deal with more wretched in ruins of old Silvermoon
go to malfunctioning sanctum which was built(west sanctum) clear out the mana creatures. Find stealthed night elf sentinels. Kill them, find their orders. (Which as posted before state to sabotage sanctum)
Go kill their leader which is the dwarf who the Blood Elves thought was there to negotiate the elves rejoining the alliance (after they left because Garithos and yes he was in command of the alliance in TFT).
Then a bunch of things with more wretche, murlocks, Amani trolls, stupid students, finding the dead scar is permanent and heartbreakingly have to kill tree ents who the elves have had good relations with because the ents are desperate to rebuild a forest, but don't realise the Elves can't allow it due to scourge likelyhood of using it to sneak into Eversong. (And couple other things I missed out.)
Ghostlands:
This is where most Blood elf players see their first horde, a Forsaken woman who sends you off to cure an ailment on a Farstrider.
Tranquillen where you find Sylvanas has sent her own forces to help the Blood elves against the scourge, no reason except for how she feels about her old people. Also get quests to go over North and West Ghostlands. One to clear a Nubian held village. Another to kill scourged controlled ghosts of highelves, due to the scourge tactics. Find a Night Elf raiding party with plans to cause more damage to the still neutral Blood elves. Plus dealing with dead scar.
This builds up to clearing Windrunner spire. You get Sylvanas pendent that was a gift from Alleria. Sylvanas pretends to disown her past life but her true self opens up (Lament of the Highborne.).
Back to Ghostlands, fight Amani trolls. Then you do the Deathholm stuff. Kill Dar'Khan Drathir. Go see Thrall who sees that the Blood elves can be worthy of the Horde, plus the Blood elves link to outland means both the Elves and Horde need each other. Then you go running around the 20-60 zones. As you just brought the Blood Elves into the Horde.