How to fix blotchy deck
Check the local hardware store paint department. Behr makes a series of deck products that are very good. They include a wood conditioner and mold remover, as I recall. These blotches are very hard to get rid of completely. I ended up restaining a deck once with a solid stain product to hide the black discolorations. Deck paint also works of course. I would investigate the latest wood deck reconditioner then stain or paint a solid color.