Blodfresychen is a cauliflower, blodfresych is cauliflowers. It is not a substance such as water, flour or coffee, and it is countable.
Why not "I want cauliflower" eg answering "Do you want cauliflower or cabbage?"
It is more of a "noncount noun" like milk Than a "count " noun.
It depends on context, I guess. If someone was serving your dinner, they'd say "would you like some cauliflower?" but if you were in a shop, you'd ask for "a cauliflower".