The book is about a boy who comes to realize that his father is a dark angel/elf who used his mother to get a new elf/angel into the world. When he grew up, he was taken by a certain elf/angel, named Claudia. I remembered they said her wings were dark purple to silver at the top of the feathers (their wings are like angel wings, but I don't know anymore if they were elves or angels).
To gain his wings the boy had to train his powers first, with Claudia, in a cave with crystals, where other elves/angels lived. He had to try to summon his weapon. After that, he had to do a mentally difficult test to see if he was like his father, a dark elf/angel or a good one, with colored wings.
The book was written in Dutch.
I also remember some part of the cover of the book:
It was a grey wrinkled background with two black wings on it. The title was written in red I believe. Maybe in very sharp letters, I'm not sure about that.
I read it in 2013. But I have no idea if it has been translated in English.