The volatility of the cryptocurrency market makes it a risk for banks to let customers buy bitcoin and etherium on credit; it's easy for someone to buy more in crypto than they can afford to pay on their card. If coin valuations take a sharp downturn (as we've seen with bitcoin's fall this year), so too can the bank's chances of recouping that money. A LendEDU study last year showed that 18 percent of bitcoin buyers snapped up the digital currency on credit, and of those, 22 percent could not pay off their balance.

That's not to say you can't use a credit card to pick up a prepaid debit card and buy bitcoin that way. The block won't really stop anyone from buying cryptocurrency who wants to throw away their money get in on the craze. But it might cause some to reconsider whether bitcoin is a smart investment.