Matthew J Ghio says:

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Is there a way of marking digital coins, so the depositor can be identified?

Only if the depositor marks them as such. The depositor generates the coin, and gives it to the bank to sign it. The bank signs it in exchange for another valid signed token. The bank can't alter it in any way, the bank can only sign it.

You have the protocol reversed. It is the person who withdraws the money who blinds a coin. Depositors can indeed be tracked by collusion between the person minting the coin and the bank, but in practice this would not be a problem. Perry