A genotype includes all of the genes in the betta's DNA and a phenotype is just the genes that are expressed, what you can see in other words. So, a non-melano female might have a genotype for melano offspring, but not a phenotype (she won't be black herself). Melano females are infertile, and therefore the color has to be perpetuated using melano 'carrier' females of other colors. To make sure you get a female carrier, you have to breed a melano male with a multicolor, then breed the offspring, some of which are carriers of melano, but none of which will appear melano. Thats how you get a melano; basically you have to start with a melano to get a decent proportion of melano fish.
I read that black lace females are actually fertile, so when you cross a melano male with a black lace female thats how you get the double black. But I dont know for sure what the percentage of the offspring would be. I suppose you would get more multicolor and melano offspring than phenotypical double black fish because multicolor trait is more dominant than the melano trait, I would guess (based on the typical 9:3:3:1 ratio), around 6 double blacks in a fry batch of 100. Hope this helped.