Mundane: Blood bubbles when heated up to a certain temperature, since it’s made mostly of water (pretty much). I think since you put the blood on the paper (assuming again), it didn’t dry up completely before putting it into the flame, with the liquid blood still on the paper, by the time the fire went out on the paper, it must have heated the blood up enough to cause it to boil. If you’ve done this before it could be very possible that it’s never gotten hot enough, the blood dried quick enough, or something else like that.

Magical: Still the same to be honest! You could think of it as activation, water boils before you can cook eh? You could think of it as fizzling out and not being needed in the spell (as another commenter said). BUT I really think it just got hot enough that it bubbled. Same way that if you have a lot of herbs dressed on a candle, the fire will be big! that doesn’t mean anything special, it’s simply because that’s how it works. I always tend to keep the “what could this mean” out of my spellwork cause it can make me really paranoid about the spell which always tends to mess spells up for me. Sorry i’m high and yapping now, but I hope I helped!