Do note, I'm not talking about Blue Ice here.
Steps to repdoduce
- Place a water source block in a cold biome,
- let it turn into ice,
- make a 4x4x4 box around the ice block and yourself with no windows or openings (so that it should be extremely dark in the box), and surprise!
The room would be dark when enclosed.
Free sun light! For some reason the ice block keeps the inside of the box from getting dark.
Break the ice block to confirm that there is light coming from it, and then all youll see is the darkness you should have saw to begin with.