User blog comment:B214/Volume 22 spoilers/@comment-201.215.165.7-20170213165134/@comment-31137559-20170214235901

Think about it like Harry Potter. Harry is considered a pureblood wizard although his mothee was muggle-born. While the species you are doesn't change upon being reincarnated into a devil, but the Underworld's standards, you are a devil. Reincarnated devils are devils who weren't origibally devils and werw made into devils using the evil pjeces system. If Issei and Rias' children aren't pureblood, there would be entirely no point to the reincarnated devil system. Issei is a devil and despite how the higher-ups feel, he is a devil.

Vali is half-blood because his father's a devil and his mother is human. And that's how Vali got Divine Dividing. Both species have to be different for the child to ve half-blood. The only children that will be half-bloods are Irina's, Kunou's, and Elmenheide's. We can talk about what kind of hybrids Issei's children will be, but it won't change the fact that while Issei is a dragon, he is also a devil.