Mass creation and manipulation, as well as some degree of light manipulation.
The saddest (or best for the victim) part about this cool concept is that shadows have no mass. Shadows, as well as penumbra, are simply the effect of reduced or absent light. In this case, the Control of the shadows is nothing but an intricate game of mass manipulation and light bending, with the supposed shadow absorbing most of the light that reaches it, thus giving the impression of a lack of light and the characteristic faded/black coloration. The darkness generation and an eventual blinding ability are but the reflex of bending the light so that it doesn't reach certain objects or structures (be it the floor, a wall or the human eye, thus explaining why everyone seems to go blind when the character desires), with the mass creation and manipulation kicking in immediately after.
Summing up, your character can't control shadows, that's basically impossible as a shadow is but the absence of light, BUT you character can in fact control and manipulate matter, as well as light, giving the impression of the shadows being controlled . Just be warned that maintaining the illusion of shadow manipulation will require focus from your character in order to use both abilities usually simultaneously and in a synchronized way.
Regarding potential secondary abilities: if your character makes it so all light is completely deflected from them, you'll be able to achieve invisibility (you will be functionally blind though, as no light will be reaching your eyes. You can also use the matter creation and manipulation ability to enter buildings by opening the walls, making weapons and obstacles, creating holding spots to allow them to climb buildings, potential flight (if you can make the created matter levitate at will, just look at the Manga version esdeath from akame ga kill), creation of golems/minions (you'll need to control their every movement though, as they're brainless and not alive, so it will be a mentally challenging task), blinding your opponents and influencing cancer to some degree (if you can also control the UV light spectrum). There may be many others I can't think of right now.
Sadly, neither of these 2 secondary powers can be satisfyingly explained by modern science as far as I'm aware of.