I don't even know if Delta is a "Participating Carrier" for the miles-for-baggage purposes - but this part of the Flying Blue T&Cs would suggest you cannot use miles to redeem "extra" baggage (and given that DL gives no free baggage on domestics, I presume a single bag would still be considered "extra") on Award Redemptions:
3.15 Awards in the form of extra baggage allowance is the extra baggage allowance above the normal allowance in the class paid for or the Member’s status in the Programme as further defined in FB Communication. Extra baggage awards are valid only on eligible Air France, KLM and Participating Carriers flights. Award tickets do not qualify for an extra baggage allowance Award. Extra baggage awards will not be accepted for exceptional luggage or for transportation of animals.