Regrouting the whole pool is quite an undertaking probably best left to a professional. I would think about that for a long time
I can help you reduce or possible get rid of the black though. That's black algae and grout lines are one of it's favorite place. It appears because of a CHRONICALLY low level of chlorine.
To remove or reduce it will take a spray bottle of undiluted clorox and lowering the water level below the grout lines. if you want to tackle that, we can describe in more detail but it is not a 30 minute task........it will take you awhile. Post back if you want more info.
Your current chlorine levels will need to be raised and held raised forever or it will come back.
