The Agouti gene ("A") is the gene that shows up the pattern on a cat, it makes the lighter coloured hairs in the background of the tabby pattern, therefore revealing it.
It is a dominant gene.
The gene of "melanism" as applied to Bengals is the Non - Agouti gene ("a").
If a cat has two copies of the Non-Agouti gene(a) instead of the Agouti (A) gene, it is a self or solid coloured cat. Because the brown spotted Bengal is basically a Black tabby cat, the Non-Agouti cat has a black tabby pattern on a black background so looks like a black cat, ie a solid black cat. This is what many refer to as a "melanistic"
Two snows can only produce snow kittens, but with the addition of the Non- Agouti gene instead of the Agouti the snows then have the colour -pointed Siamese pattern, ie darker solid heads and points.
They may or may not have noticeable spots on the body, but like "melanistic" blacks they would probably fade as the cat develops its adult coat, but I have no experience of them so cannot say that for certain.
The Siamese look for Bengals is not one which Bengal breeders want on the show bench therefore one of the reasons that many breeders do not want to officially accept nor disseminate the Non-Agouti gene.