50 is good for a crop sensor cam... Most people shoot bust style portraits at 85-135 so you are compressing the space a little, and not dealing with distortion in the face of your subject. On a crop sensor body, 50mm = ~75mm
Portrait generally just refers to a photo where there is a person that is intentionally the subject. It can be a bust (shoulders, neck, head), which is the traditional concept of a portrait:
Or, a portrait can include the situation at hand, allowing you to experience a little more of that person, who they are, what they are about, what they are doing... but still focuses on that person and how what else is going on around them is adding to your experience of them
(I saw the most awesome portrait of a syrian rebel the other night, but can't find it today)
So, calling something a "portrait lens" is sorta silly to me... there are some lenses with wonderful bokeh characteristics and focal lengths that are helpful, but a portrait isn't really defined by a focal length, you know?