On the other hand, I've been using bloodswels for leveling ever since I unlocked B2P. If you can spare the blackspace, and it's totaly worth it, it gives more fireballs than Clearance does. Especially with just about any subdungeon.
Just count it - 2 fireballs from your mana pool, 3 if wizard. 2 more from a regular health pool, 2 more from bloodswell, and 1 more from CB. That's a lot of fireballs. Throw an IMAWAL down before the fight, save up a few charges on CB, find a soft lvl8 or 9 and procede to the boss fight... Just remember to pick stuff that you revealed up, since the routine also does exploration for you, and set up a nice ding for the boss fight...
Oh, forgot to mention that if you're going this route, you picked up lifesteal rather than sanguine, so you can probably count on one more fireball from munching on random popcorn before you throw the IMAWAL down... Price? a bunch of drac piety, 5 gold, some blackspace and a walled up popcorn.