My question is when you put in the Black Algaetrine? According to the MSDS on it it contains copper carbonate (I believe it's 3% copper) and some organic ammonia compound (MONOETHANOLAMINE, TRIETHANOLAMINE) which form choramines(that's combined chlorine!) when put into your pool,and DIMETHYL BENZYL AMMONIUM CHLORIDE which is a quat (NOT polyquat!) . The choramines are eaten by the algae and decolorize them. The quat and the copper both are algaecides. This can create a very big chlorine demand. Have you tested your water for copper and iron? The black in your pool might not have been algae at all but a copper stain. In a highly oxidized state copper will create black stains! This is a complex situation but it can be fixed. IMHO, the first step is to see if there are metals in your water and to test the stain to see if it is a metal stain. There are stain tests kits available or you can determine what the stain is with the help of some of the more experienced people on here! First thing is to determine if it is really a stain and what kind or an algae problem . The treatments are different depending on what the results are.
Don't loss faith. You will be swimming soon!