Airlines normally don't do this. Once miles have been credited, they have been credited. Everything else would just be unneccessary administrative work.

That said, you can give it a try. Write a letter (!) to Flying Blue in which you explain that, due to your travel agent's fault (if you used one) or whatever sounds like a plausible reason, the miles ended on the wrong account and ask them to delete the miles.
If you are lucky they will do it, but you have to be lucky.
Once FB has deleted the miles, send a retro-claim to DL's program and include a screen shot from your FB account that shows the miles were first booked and then deleted.

The difficult part of the procedure is getting the miles deleted. The retro-credit part is pretty much standard procedure.

Good luck.