I don't know whether Kamaka and D'Addario strings are exactly the same. I think there might be a tiny difference in gauges, but as I said they feel and sound identical. Nylon strings in general seem very similar regardless of the manufacturer. I'm sure the big string manufacturers like D'Addario or GHS produce strings for other brands like Kamaka or Ko'olau who then repackage and brand those strings. It's the same thing with D'Addario Titaniums and Ko'olau Alohi strings. They are extremely similar, quite clearly made from the same type of monofilament material, and I can't tell them apart.
I'd say the reason why people often replace Kamaka and Ko'olau strings is because they just don't like nylon strings that much and I understand why. Nylon strings usually sound quieter and lack the projection of some of the more modern strings available these days. What they excel in is a richness and warmth in their tone, and they also feel different because of their larger gauge. The trend, in my opinion, has been towards thinner and brighter sounding strings, but personally I really like the warmth of nylon strings and I massively prefer the feel of them to any other type of string. If you also like those types of strings, don't be afraid to try them on your ukes. Just be aware that, at least in my opinion, they don't suit all kinds of ukes quite as well as fluorocarbon strings. Particularly the purple Titanium strings and equivalents are very hit and miss in my experience and are better suited for larger bodied ukes that already have natural depth and warmth in their tone.
And just as a side note, Aquila strings aren't really nylon but rather nylgut, their own proprietary material. Their sound is noticeably different to nylon strings, usually louder and a bit brighter but very distinct which results in many ukes fitted with nylgut strings sounding the same.
Regarding black strings, outside of black nylon your options are usually black/brown fluorocarbon, so Fremont Blacklines and Worth Browns, aNueNue Blackwater strings if you can find any. Can't think of any others of the top of my head.