Seal of Blood isn't at the trainer.

My paladin trainers don't sell Seal of Blood. Guess I can't do dps

Make sure you're visiting the appropriate trainer.

I've visited all the trainers in silvermoon city. is there a special trainer somewhere that teaches Seal of Blood?

Edit: my bad i think it was wotlk

there is only one rank of that Seal in TBC so just use the Rank1 from ur spellbook