"Blockchains can prove that two parties once agreed on something, cheaply." - well actually you can do that with electronic signatures, much cheaper than with b

"Blockchains can prove that two parties once agreed on something, cheaply." - well actually you can do that with electronic signatures, much cheaper than with blockchains. The problem that blockchain solves is a very specific timestamping of messages between many participants. For two participants just signing a date is enough - the problem is if A singes two checks to B and C and you want to know which one was first.