The timing is a bit off because this was written in 2018 not five to ten years ago, but could this be The Magician's Brother by H. D. A. Roberts?
The protagonist is Matthew Graves and his magic is literally dark:
Every magician has a specialty; for your adept (the lowest rung on the magical ladder), it's often little more than a single spell, lighting a fire, for example. For your Wizards and Sorcerers, it can be an entire element or even two. For the less powerful, that's often all they have, but your stronger mages are able, with enough practice, to cast a wide variety of spells, as long as you have the skill to craft it and the power to charge it. But your specialty will always come easiest to you.
For me, it's Shadow.
Otherwise known as Umbrakinesis. Now, that sounds fun, doesn't it? The very darkness itself jumps up and does what it's told, what could be more fun? Very good questions, and of course I might well agree with you... if I wasn't afraid of the dark
It is set at a school, and there are magic duels. The book starts with the protagonist's brother Desmond practicing in the gym for the next competition (and getting a kicking :-).
My brother threw a fireball. His opponent raised her hand, and a single gesture sent it ricocheting back towards him. He waved his hands frantically, trying to dispel. Idiot. The ball was moving too fast, and his counter-spells have always been too slow. He should have dodged and cast another attack spell; his only advantage in the match was his raw power.
Belle was just too quick and far too smart to fence with. Ah, he's got the fire out and... Belle hits him in the face with an ice-ball. Desmond's shield glowed, but enough force was transferred that he hit the floor (with an amusingly solid thump), match over.
Matthew does use an invisibility spell, but it's not to steal a book. It's a remote observation spell:
I sat down carefully, fully releasing my focus into the spell. It was a remote observation enchantment, and quite a complex one, combining about seven smaller spells into the whole, not including the light refracting shell, which functioned as basic invisibility (to my frustration, I hadn't been able to figure out how to make it work on a human body).
He gets caught by two villains and his magic skins them alive - I did say his magic was dark!
Matthew doesn't kill his brother, but there is a conflict between them because Matthew is Shadowborn with his brother is Lightborn, and there is a natural antipathy between the two, especially when they are brothers.
I flicked through. It was an essay type document, with references and points by section, dealing with each case in turn. It confirmed what Kraab had told me, and my blood ran cold as I read on. There were, in fact, six records of twins like Des and I. All six Shadowborn were murdered by their siblings. The author of the document even went so far as to theorise the existence of a curse on such brothers, due to the consistent end result.