User blog comment:B214/Volume 22 spoilers/@comment-95.90.227.198-20170201191840/@comment-29963364-20170203185333

Ophis didn't give them any power (like she did for some members of the Khaos Brigade).

She used a bit of her power to call forth (or unlock) their hidden potential (like she did for Asia and Irina).

She essentially acted as a "catalyst" to draw out the full power within Issei and Vali. That's why the DxD G and DxD L have black in their color scheme.

She didn't give them her snakes, so if that's not "blessing," then I don't know what to call it.