If it looks like it's got lots of internal cracking going on, then the "crackle" might be referring to the stone having been heated to produce said internal cracking. You might be able to find pics of "crackle quartz" and "crackle glass" to get an idea of what those look like; in both cases, the internal fractures were manmade.
Agate is a stone that is frequently color-enhanced, in order to make it more appealing for sale. There are some natural blue agates, for example, like Blue Lace Agate, but if you see an agate that's a super-vibrant blue or purple, that color is most likely dye. Normal agate is a plain greyish-brown shade a lot of the time, and while crystal lovers and geology-oriented stone collectors will find that just fine, the vendors are looking at the much wider general market, who want to see pretty rainbow colors. (This is the same issue I've run into in museum gift shops: the target market is kids, and kids like bright colors, so a lot of the stones in museum shops are dyed.)
Anyway, I'm not sure there will be a listing out there specifically for BLACK agate, since that's another color that can, at times, be added by human hands to bring out a nice contrast in the pretty banded patterns. (And I'm not saying there's no natural black agate -- I haven't seen it, but Mother Nature is pretty creative with her stone colors so it could certainly be out there.)
All this yakkety-yak boils down to, you will probably be able to use a combination of the properties of Agate (which is a member of the Quartz family) and the properties of the colors of the stone (as I'm a proponent of color therapy). If you feel that there are man-made alterations to the stone, and you're not sure the vibes left behind are helping matters, then you can do a targeted "clear" to wipe out human-induced energies while keeping the beneficial natural energies of the stone itself. All members of the Quartz family have lovely healing energies, and black is a nice, solid, grounding color.
Boy, am I a chatterbox, lol. I hope this helps!