Agents of STRIKE
The Black Widow program appears to a specialized Skrull breeding and super-soldier project, where human-Skrull hybrids were intentionally created and trained from childhood to serve as spies and assassins for Leviathan.
Black Widows are born human in appearance, and trained from childhood not knowing of their true heritage but raised as loyal to Leviathan and trained as child soldiers and ruthless combatants as soon as they can walk.
When they reach adulthood, they are subjected to gene therapy that causes their Skrull genetics to express, causing them to become more or less genetically identical to full-blooded Skrulls, complete with the full suite of abilities that one would expect from a Skrull coupled with a lifetime of training in espionage and assassination.
If they do not receive that gene therapy, they will continue to live their lives essentially as indistinguishable from humans, with only specific genetic markers identifiable if you know how to look for them.
Thus far, all Black Widow subjects we have encountered have also been sterile. It is unclear if this is because Leviathan intentionally sterilizes the subjects, or because like many cross-species hybrids like mules and ligers, human-Skrull hybrids are born sterile. More research is required.