Was Wondering What Causes This Kind Of Colour To A Silver Dollar ?

Very heavily patinated with the sulfides and oxides of silver, and to a lesser extent, the sulfides and oxides of copper - all of which are black.

"Don't lean coins !!" the seller has obeyed.
It looks very grotty, being as black as it is. Very much down on eye appeal.
IF I had bought it, with some circumspection, perhaps I would have considered giving a series of very quick dips with an acetone rinse in between each.

I have an example already of one of these, in blast white proof like MS64 or 5. Looks much nicer.

I love this type of coin. Because of the proof like polished finish this issue seems to tone like this sometimes more so than other silver dollars. Sometimes a man just gets tired of seeing bright white coins. Though common, what a Great coin

Too many coins not enough collectors anymore it seems. Good time to buy, I suppose

Looks like natural, terminal toning to me. Under bright light sometimes these show a beautiful array of subdued hues.

That's exactly what happens to Grandma's tea set if you don't polish it. The tarnish depends on the storage condition. That coin is so tarnished that a dip will not take it all off. Here's a 1956 from a PL set that was just on the edge of solid black. The underlying surface has been damaged and shows a dull brown after dipping. The obverse came out better but still has a brown tint. Then a 1957 that had splotchy dark spots and a fingerprint. Dipping can also show marks that you couldn't see when they were tarnished. Sometimes it works, and sometimes not.

Out of 4,500 coins in my collection, I have only dipped two. Both silver, both below F15.
Pretty much, I could't be bothered with dipping.
The advantage of dipping is that there is no mechanical rubbing involved.

Nevertheless, I have seen dealers (only very rarely), give a quick dip to proof silver coins. Results looked to be reasonably favorable. Experience needed.