真っ白 and 真っ黒 pure white and pitch black

Last week I bought a new pair of shoes. They are 真っ白. My shoes are PURE WHITE. (But they will get dirty soon I think!)

I was walking through the park one evening and it was very dark. I heard someone say 真っ黒です. In English we say it's PITCH BLACK. 

Why aren’t 真っ白 and 真っ黒 the same in English?

真っ is an adjective, it means totally or completely. We use PURE with colors in English. Pure red means completely red. But when we talk about how dark it is (闇) we use the expression PITCH BLACK.
You mean pitch?

No no. Pitch is a kind of tar used to fix roads or holes in a roof and it is very black. "Pitch black" means it's as dark as pitch.

"We lost power during the thunderstorm. My house was pitch black."