I do not know whether the block is supposed to be translucent, or be a regular block affected by light levels. No matter what dimension you are in, or under what light level the block is placed at, it does not cast light or get affected by light.
Environment
Windows 7 Desktop Computer, 8 GB RAM, no mods or third-party software, singleplayer.
Actually, the magma block does cast light. It's light value at max is only 2. Take a look at the light levels in the pictures I posted.