People are curious, it's human nature. They want to know why a brown skinned girl talks with a London accent and says "England" when you ask her where she's from.
They want more information. They want to know why they can't pidgeon hole me into any one particular ethnic group just by looking at me. They want to know where my parents are from.
I don't volunteer this information unless specifically asked. Mostly because it's none of their business. That I'm English should be enough information for the 2 hours or days I will know you. But, if they ask, I will usual tell.
However I've never had the question worded in such a unique way before. "What is your blood" left me confused for a minute.