You can still DOS free-of-charge in the stage of providing the outputs. You can't identify the signers of the inputs to know who to charge a fee until the last stage where all inputs need to be signed to the transaction.
By definition the blind signing of the outputs has to be uncorrelated with the inputs.
http://www.darkcoin.io/downloads/DarkSendDocumentation.pdf#page=2Stage 2: Collecting Inputs
Stage 3: Collecting Outputs
Stage 4: Collecting Signatures