Note that Koth does not work the way you think he does with Blood Crypt . Given he turns the mountain into a creature, it gains summoning sickness till the end of the turn if you just played the crypt that turn. Therefore you'd either have to use the mana on another player's turn, or wait until it untaps normally.

You're absolutely right, he isn't immediately able to turn Blood Crypt around for damage. However, I can have it (essentially) untap the turn it comes onto the battlefield.

From the Magic: the Gathering Comprehensive rules:

-605.3a A player may activate an activated mana ability whenever he or she has priority, whenever he or she is casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment, even if it's in the middle of casting or resolving a spell or activating or resolving an ability.

Blood Crypt doesn't lose its ability "Tap: add B or R to your mana pool" when it becomes a creature, so I can activate its mana ability on the turn it enters the battlefield even if it is a 4/4 Elemental.

That last comment was me. If my username is showing, disregard this post.

It does not lose that ability, no, but upon gaining summoning sickness, it cannot tap for any reason. The same reason that mana dorks can't tap for mana the turn they come into play.

Sorry, you're right. Let me change that. :'(

Of course, hon. :3 I just would hate for that to be a make or break situation for you down the road.

Yeah...it's still a good ramp card, and I'm saving up for a Badlands .

Indeed it is, and luckily because it only stays a creature until the end of your turn, it will be able to be tapped during your opponents' turns.