In the same fashion of this question, you can't mine Crimstone Block prior of making a fifth tier pickaxe (Deathbringer pickaxe in your case, which is the Crimson counterpart of the Nightmare pickaxe). Thus you're right in saying that you need Tissue Sample, which is looted on the Brain of Ctlulhu.
So in order to progress, you'll have to break the Crimson Hearts (The Crimson counterpart of the Shadow Orbs). Since you can't mine the Crimstone Blocks yet, you'll have to use explosives in order to get to them: they will explose the blocks in your way, then you can destroy them with a hammer or explosives.
Then you'll be able to craft the fifth tier pickaxe and progress in the game.