I read this book approximately 10 to 15 years ago; here is what I remember. Might not be 100% but should be close.
It was about an expert hacker who learned to hack DNA code.
He was a twin but his sister died when they were younger. She died when they were kidnapped as children.
The hacker had blue eyes and "blue" might have been in the title. The book had a female police officer trying to track him down.
Most of the book was about a genetic disease that transferred person to person swapping DNA from everyone. The hacker made it customizable with animal DNA. He could give someone a custom-made version of the disease so they could have animal traits like fur or feathers or eagle eyes. He gave himself fangs under his fingernails for defense but for others it was mostly cosmetic or fun stuff.