Planet Money did an episode on exactly this topic

There have been some developments since this episode.

To answer your question: Too much cash is a big problem for Colorado pot sellers. And banks have not been taking cash from pot businesses because it puts them afoul of federal banking regulations. But the feds (i.e. the current administration) have recently told banks they may in fact accept deposits from known pot sellers and not worry about running afoul of the law. They have rewritten the regulations to this effect (which I think actually involved an act of congress if I remember correctly). They were concerned about public safety vis-a-vis so much unsecured cash floating around so they lightened up the rules. Last I heard most banks were still very reluctant to accept deposits---mainly worrying about a even a small risk that the favorable disposition of the feds could change with the next election---but it's theoretically legal now and inevitably some banks are going to start accepting deposits. If not currently extant banks someone is going to open one up soon enough (if they haven't already).

As far as cryptocurrency goes: contrary to /u/Crassusinyourasses assertion, if done right it could be used to hold assets and be well nigh impossible for the feds to trace. It wouldn't be easy to set up, you couldn't just go to the usual exchanges, but it's possible. But even if you used the usual exchanges, which would reveal your identity to anyone looking at the block chain, you could still make it hard for the feds to seize so long as you were willing to commit perjury to hide it. You could physically hide the crypto-wallet just like cash (even in paper form).

But just from the standpoint of volatility bitcoin is terribly risky: it's value could drop dramatically when you want to cash out and may never return.