It's a digit in hex format between 0 - F so in decimal between 0-15.
I know. I asked about the randomness of its appearance as the last digit of a block hash. Whether increasing difficulty has any effect on it etc.
Your chance to guess the right digit is 1 in 16.
That's not always true for this specific game as the a player can chose multiple options at a time.
It is random as long miners/pools don't care about the site.
If the bets become bigger than subsidy (25BTC), it gets interesting.
Miners/pools can participate on site, rig future block digit to be zero, and sweep all unsuspecting user's funds who bet on 1.
I doubt if there is slightest truth in the bold part. Mining pools probably can not force the last digit of a block hash to be zero or any digit of their choice. It should depend on the nonce. Otherwise SHA would have been broken.