Salem

Is there a trick to it, I have 2 mines that I've been working at .. Find lots of Copper & Iron but no black tiles for cave down to the next level.

https://imgur.com/a/hepBV

No trick, everything is random. It's a bit unusual to not find any black in that large an area, but they're there. You should probably mine in some long straight lines to maximize your chances.

I was hoping to have the mine-down close to the ladder so that I didn't have to walk as far between trips..

Also , I understand there are 8 levels, I was hoping to find some 50% or higher iron, any idea what level I would need for that?

With a very mediocre +mining suit (max-1 slot clothes, no amethysts), you can get into the low 50's near the corners of level 6, and not far from the ladder at all on level 7.

so mining skill = black walls? My character currently has 165 Mines & Mountains, not wearing mining gear

No, I was talking about the potential iron purity.

I believe JC said in the past that the only things that affect black-tile chance are the time the level has existed, how much has been mined, and how much has been mined each day.

According to him, iirc, the best way to find black-tile is to mine a little bit each day, because the increase in black tile chance has diminishing returns each day (e.g., mining 100 boulders could have a big increase over mining 20, but mining 500 boulders might not have a big increase over mining 100). He seemed to not want people "brute-forcing" deep mines, though you still can if you're willing to slog through it, especially with a little luck.

oh ok thanks, I have a WAYS to go then ..

No, black tiles and all other tiles are generated at the time the mine level first appears. It's the chance of a black tile triggering a cavedown that goes up based on time, level of the mine, and other factors.

What is the reasonable distance between tunnels to seeking Black tiles? I suppose about 6 because black ones are not single. Am I right?

I do 3, but thats because its more reliable at finding ore as well.
6 is fine as well.