Well, it's good that you were able to get some of the miles into your Delta account.

But doing a retro-claim like that, online, will not cause the miles to be taken out of your Flying Blue account, and put into your Delta account. When the system verifies that those miles have already been booked elsewhere, you will just get an error saying that the miles were already granted elsewhere.

Just out of curiosity - why would you have both a SkyMiles and a Flying Blue account? There may be limited cases whereby it's worthwhile collecting in both schemes (i.e. for people that fly enough to easily gain status in both) but for most flyers, it's counter-intuitive to have more than one frequent flyer programme account in the same alliance.