Immediately following the injury, the cause of dullness now could be an optic nerve inflammation or an entity called Commotio retinae. They do reslove with time but it could leave behind a visual manifestation where the vision is fine like you have but colours appear a little washed out . When you say your eye appears smaller, this too could have a few reasons. The one that seems likely is a mild trauma or perhaps a disinsertion of the levator muscle that raises the upper lid. If its just mild it does not cover the pupil and you may not have a vision related issue. I am assuming the blow you sustained was not severe enough to cause any bone damage in that area and no treatment was needed. If any other muscles of the eye were damaged, you could experience some double vision but that is probably not the case since you have not mentioned anything of the kind. If everything is stationary since then, you do not need any treatment. If you need to know for sure, an eye exam to check for the levator function and a funduscopy to look at the retina for any old damage can be done.