I don't think the fungibility of Dash necessarily depends on which proportion of transactions are darksend transactions. The traceability of funds depends on the number of rounds of darksend mixing they go through. The more rounds of mixing, the more difficult it is to trace. If you receive funds which are then "blacklisted" you can always put the questionable funds through as many rounds of mixing as you want, then it will be impossible to say for sure that the funds that you hold are the blacklisted funds, as they will have passed through many different addresses in many different transactions.

One important difference between Dash and CryptoNote mixing is that with Dash it is done off chain by the Masternodes, as opposed to the on-chain mixing CryptoNote currencies use. I don't know enough about CryptoNote to go in to it in much detail, but I believe that while it may be impossible to trace transactions today, in the future the technology may exist to trace all mixing which has ever happened as it recorded in the blockchain.