If the IRS is suspicious, they might ask who did you buy it from. If you don't know, but you can show that when you bought the $500 btc earlier in the year, that was the going rate for 1 btc, then later about $1200 was the going rate for 1 btc, they will probably accept that. I doubt they will ever care about a 'blockchain' or anything like that.
It will take cases brought to court to answer the question, but I think IRS auditors will accept a record of a transaction on the block chain as a valid receipt. Paper receipts are easily counterfeited. A knowledgeable IRS auditor will realize that a confirmed blockchain transaction is a much harder receipt than a paper receipt. They may want confirmation from the receiving party before fully validating it, but they definitely won't be asking the coffee shop.