Blodfresychen is a cauliflower, blodfresych is cauliflowers. It is not a substance such as water, flour or coffee, and it is countable.
blodfresych actually refers to a group or bunch of cauliflower (just as bresych is a group or bunch of cabbages). I think blodfresychen would refer to a single head of cauliflower.
In English you rarely say 'a cauliflower', just like you don't usually say 'a broccoli'
Why not "I want cauliflower" eg answering "Do you want cauliflower or cabbage?"
My neighbours on Gower grew excellent cauliflowers. Note plural! The Normans grabbed that part of Gower soon after the Conquest, so it was English speaking. If I wanted a cauliflower, I asked for a cauliflower. If I wanted two, two cauliflowers. (Actually 2 caulis!) For broccoli, 'a head of..' or 'two heads of..' At a meal we would say 'some cauli or some broccoli'.
I agree milk isn't countable - unless in bottles, but I can count cauliflowers, cabbages etc. They only become uncountable when turned into dinner servings.