Hi. Since Illustrator is Object oriented, not pixel oriented, there's no direct option to select the inverse like in Photoshop. However, remember, that letter IS an Object - so if you put a black rectangle on behind it, it'll have black wherever that letter is transparent. There are several ways to draw behind that object. You can select "Draw Behind" (https://helpx.adobe.com/illustrator/how-to/draw-content-behind-and-inside.html), but an easier way would be to just draw a black rectangle, right click it, and select Arrange>Move Backward.
IF you want to cut the shape OUT of a black rectangle, make sure the shape is in front of the black rectangle. Convert you letter to Outline (Type>Create Outlines). Select both the letter and the rectangle, and in the menu, choose Object> Compound Path>Make