As a 44 year old dark skinned woman who is experiencing this, I don't think sun exposure is the initial reason. I hardly wore shorts or exposed my body growing up (very modest, plus didn't think I was pretty as a teen) but I had an IGH spot on each calf, several together on my waist (where the band of panties come to) and several together between my breasts (never did bikinis or low V-necks). So I was used to theses discolorations.
When I hit my late 30s, I started seeing a few on my arms and now in my 40s the IGH started to behave "actively." I have now some on my upper front thighs, face and upper shoulder regions, from speck to millimeter sizes. Some are raised and some smooth like regular skin. The dermatologists I've seen are dismissive but I will keep on researching. As a person who doesn't wear foundation makeup, I don't won't to start because of hiding mismatched skin. I am experimenting with Retin-A on just one of my facial spots to see if change will occur.