Pool is clear but bottom looks spotty and dirty

OK, so another option for you is to use Trichlor either as tabs or granular. The most common black algaecide is in fact Trichlor granular. The high chlorine level and low pH combine to be potent, BUT you can only use this on a plaster surface as it will destroy vinyl. Unfortunately, your concrete pool is painted so it would likely bleach out the paint, but if your paint is white then it might be OK. You could try a little in an area you don't care about and see if it leaves a mark. Of course, this method may work on the floor, but not for the walls.

Are you positive you have black algae? If you are able to scrape some off and spread it on a piece of paper, it should look dark green and not black. It is possible that you might have some metal staining instead. If you have metal stains, then lowering the pH of the pool would lighten them up. Also, putting some ascorbic acid (Vitamin C) would remove the stain if it were iron (if copper, then only the acid would lighten it).

It is black algae, I can scrape them and it is dark green, it looks tar-like when it is at the bottom and has tough texture and if I remove with a bit of force (from where I can) they float and are like algae. As I said in my previous post I was able to remove a lot of it using the wirebrush, and they were caught in the secondary skimmer (fine mesh) and they are green, granular and plant like. but it got to a point where it doesnt go away at all after 3-4 weeks.
I will use trichlor and let you know how that goes. I have trichlor tabs, but the problem is I dont know how to apply directly on top of them especially the walls. If I get the granules I can sprinke at the bottom to see if it makes a difference atleast at the bottom so I will know it works.

Your black algae must be very deeply rooted, but that seems very odd with a painted pool. Usually, scraping off the waxy layer and exposing the deeper algae to chlorine is enough to kill it. We look forward to seeing how the Trichlor granular works though as you point out I'm not sure what you'll be able to do with the walls in that case.

I think ours is a gunite construction, not painted concrete.

OK, then you need to change your signature because it says "18x32 kidney 15K gal IG painted concrete pool". That's why I made that comment.

I thought so too.. I changed the signature now. But I am sure I tried to change it a couple of times few months ago, but it was not saving my signature for some reason then, but I was able to this time.. weird