Do you mean to change the player's server seed while they are playing?  That breaks provably fairness, since you can't know for sure exactly when we saw the new block, we can pretend not to see it for a while, and then see it at the time most advantageous for us (ie. when it changes a big bet from a win to a loss).

Or do you mean use the blockchain to add entropy to the random number pool?  The random seeds are generated using openssl's cryptgraphically strong random number generator, which in turn is seeded from the operating system's entropy pool, which collects entropy from all over.