When water flows or not, the blocks around it are darker than what they usually are supposed to be.
1. Go to a survival, or creative, world
2.Find a water source
3. Look at the blocks around it(lighting glitch not visible on grass blocks) and for a better visual, let the water flow with sand around it.
Device:iPad 2(ios)
When you say the blocks are "darker than what they usually are supposed to be," what are you comparing them to exactly?