Taxation of darkcoin when you use it with a merchant would be exactly the same and there may be that reporting thing of over 600 USD from the part of the merchant (that apparently might not be yet the case for any crypto judging from some posts).
Regarding anonymity Darkcoin uses a sort of Coinjoin technology on a public ledger. Unless it grows bigger than Bitcoin, your anonymity there will actually be substantial less than with Bitcoin.
I don't think the issue you are worried about is solvable with an alt-coin... maybe with wallet technology or third party services if you want to be fully compliant for all types of commerce and transaction sizes in the US. Or by changing the system... Bitcoin did not start without any hint of political intent. Just take look at the genesis block.
These things I think will make a cryptocoin fully anonymous
1) No public ledger
2) Totally anonymous exchange that would be on TOR and only accessible on TOR, where they can't/don't take hold of your Ip address, or personal information etc etc(also the exchange would have to be located outside the U.S.A or other countries regulating bitcoin, or just make it decentralized..)
3) the wallet itself should be synced with TOR or some IP masking offline proxy or something, making it Fully anonymous.
Those three things, no public ledger, exchange only on TOR, and the wallet being synced with TOR, I think would make the coin Completely anonymous and unable to track. Even if it is tracked by the IRS, the lack of explanation to say where the coins came from, would make filing taxes A LOT easier than having to list every bitcoin you recieved on your taxes..hope it helps a coin dev out there. Thanks.