Walls and Ceilings - Drilling holes in wall in old building

I live in a 60-year-old, 3-story, 6-unit apartment building. My wife bought some shelves that came with mounting brackets that I want to hang in my living room. The mounting hardware that came with the shelves is meant for drywall, but the wall we want to hang the shelves on is not drywall.

The wall backs up to the stairway of the apartment building, and it is very thick, and probably a supporting wall of the building. I assume it is concrete, with a layer of plaster before the concrete. I used a masonry bit to drill pilot holes for masonry screws I was going to insert to hold the shelf bracket to the wall. The first hole was solid all the way through (about 5"). On the other holes, I seemed to get about 4" and then it opened up. However, on all of the holes, black dust came out of the holes as I drilled.

My questions are:
1) What is that black dust?
2) Is is safe for me to be drilling through this wall?
3) Are these masonry screws the best way to hold the shelf mounting bracket to the wall?

Any advice is much appreciated. Thanks!

Welcome to the forums

#1 dunno

#2 should be

#3 I would think so

Are you using tap cons? I would think as long as the screws tighten up nice and snug they should do fine. Of course you want to use common sense with the weight of whatever you set on the shelves.

Thanks Marksr. I think I figured out that it's not concrete, but it's Cinder Block. And yes, I'm using tap cons, and they are working well, now that I have the right size. I had originally drilled holes with a 3/16" bit but I (stupidly) had 3/16 x 2 3/4 tap cons, so the holes were really too big. But tonight I bought some 1/4 x 2 3/4 tapcons and they are holding very nicely now.

Thanks a lot for your help.

Did you drill a hole into the chimney?