We had our first DOJ inspection last week. Our DROS are by and large ONLY blue ink, and we didn't hear any complaints.
Personally, we have done it on purpose, when auditing the contrast to the black makes it easier to see if there are things missing from the DROS and 4473.
I have gone thru every packet we have, and they are way easier to check in blue ink than black.
Your mileage may vary....