I recently asked a group of peope whether they had eaten tuna melts as a kid. Everyone remembered a version of this dish.
If a person or thing melts into something such as darkness or a crowd of people, they become difficult to see, for example because they are moving away from you or are the same colour as the background. The youths dispersed and melted into the darkness = disappear
To transit (or to change) the matter from a solid state to a liquid state, usually by a gradual heat
Hence: To soften, as by a warming or kindly influence; to relax; to render gentle or susceptible to mild influences; sometimes, in a bad sense, to take away the firmness of; to weaken
A melt is the liquid aggregate state of metals and alloys The term "melt" is often used as a synonym for the determined chemical composition, i e for a preliminary material, which is produced in one cast
{f} heat until liquid; be heated until turning to liquid; fuse; be fused; thaw; disappear