I was involved in a bit of a discussion on the whinefest idiocy of the standard WoW forums (I don't recommend being as foolish as me and actually reading those blasted things) about Blood/Martyr vs. Command for leveling.
There was a quite strong belief on the part of many that Blood while soloing will leave you drinking every other fight, and/or dead. While there have been times I've questioned that maybe its efficiency is not as strong as I'd like, to suggest that Blood rolls up devastating consequences for the Paladin unwise enough to turn it into his everyday seal is ridiculous.
For those who don't use it regularly outside of instances (and you hardly notice its damage while inside one), most of Blood's self-inflicting damage is actually quite minor. I'm taking about 50 or so damage at the worst on most swings, which adds up to far less damage than mobs are doing to me. Judgment of Light in and of itself has been more than sufficient to cover for any damage received in that method, and as such my health is never at risk with regular swings.
The real risk of Blood is the Judgment damage, which amounts to roughly 30% of the damage caused. As I've seen a 7k crit in an instance with Blood, and can regularly crit for 4.5-5k with the skill, that can add up to quite a bit, to say the least; you should expect to lose 1,000-1,700 health whenever you judge the seal.
But! Here's where the fun comes in. An Art of War-inspired insta-Flash of Light will immediately heal all or nearly all of the damage you inflicted upon yourself, and if you just crit on your Judgment, Art of War is already up anyways.
As long as you're killing mobs that don't inflict overly large damage themselves, wherein you'll need that FoL for the damage being inflicted on you and thus extra healing will be needed (pretty rare), you should find Blood to be an excellent nearly-all-purpose seal.
But still, don't PvP with it.