Photo would make this so much easier...

One top of that you got some terminology backwards: an areole is the place where branches(flowers & fruit, offshoots and sometimes even roots) come out from. Its often a fuzzy circle . Thats where the glochids are- tiny barbed spines that annoy people....

But bear in mind, opuntia also has regular cactus spines too- spiky objects found on all cacti.

I meant areoles instead of glochids :) Photo below. Thanks for your help!

I think it is sunburned. Plants, even sun loving cacti, sunburn if they suddenly get a lot more sun than they are accustomed to getting. Sunburn can cause the skin to blacken in places. The pad growing out of the flat side of another pad and growing downward seems atypical of Opuntia based on what I've seen. I'm not sure what happened there. Usually new pads grow from the top or at least the edge of the older pad.

Some succulents will turn quite dark and might look sunburned just because they are getting plenty of sunlight. However, I haven't seen cacti, or Opuntia more specifically, do that. In my experience when cacti get black skin they are sunburned. I don't know... maybe there's are some cacti that can become quite dark that I'm not thinking of but your Opuntia looks sunburned to me.

When cacti get jet black skin, i amputate and dont think about.....

Most cacti when sunburned get a corky bit.

Purple means some kind of stress....which is what im seeing here....

Based on the description in your opening post I thought maybe you had O. microdasys but the plant in your pic looks different than I expected.

If you move the plant to a spot with a bit less sunlight then it might change to a more normal shade of green if you want to try that. Its growth might speed up a bit, too.

Thanks for the feedback! I've had it in a number of different conditions and I've also photographed it in those positions. No change in coloring or structure, so I don't think it's sunburned. It's still growing steadily - just not that quickly so I'm not concerned about disease (although, there's a first for everything). I have a million cacti and I've been growing them for years - I haven't ever seen anything like this... I appreciate you taking a look - thanks!

Oh, OK. Whether you meant "a million cacti" literally or figuratively you really weren't asking for culture advice.

When I've seen sunburn it starts as a dark spot and then turns corky after a while. However, if that is happening to your plant you would've mentioned that suddenly it turned black recently. Instead you said it is always a dark color which I didn't quite acknowledge properly. Stefan's brief post made a good point.

I've been clicking through some Opuntia photos but I haven't seen one like yours. How did you get it? I mean did you grow it from seed? Did some one give you a pad? Did you buy it from somewhere? I wonder if it might be a hybrid.