I agree that Blood Weapon would have you sacrificed a little bit of HP for an increased buff but I do not agree that it would be on Grit. No one would use it.

As a tank, your main mitigation tool is your HP. The more HP you have, the better is your chance to survive the next attack. Getting rid of that HP will result in you losing mitigation.

However, for an OT skill such as Blood Weapon, I would say it is agreeable. As for Abyssal Drain, I think the purpose of the spell is also for mitigation, i.e. to recover back some health that you lost by things attacking you. It's similar to how Bloodbath works. Bloodbath only gives back 25% of your HP back, therefore the amount it heals back to you is small. The purpose of Bloodbath is to get back a percentage of HP once the enemy AA you, resulting in you mitigating some of the damage, resulting in less HP needed to cure you.