When we see stains on the pool surface, we tend to think of residual material that sat on the bottom or metal deposits that were introduced to the water. You may want to try one of the following:
- Some crushed vitamin C tabs in a sock and scrub
- Carefully apply a chlorine tablet and move gently back & forth
- Wire brush
If the black marks from cracks are from some repair work or something in the past, there may not be much you can do, but if you have more details about those cracks let us know.