Yes wet rub means Wet and Dry paper used wet.

You will have to work to hard edges. This may mean painting the whole mirror on that side which is not really that big a deal and probably what should be done.

Alternatively, paint to the edge corners only. Mask there.

Once painted you could rub the blend line and buff.

Be gentle on any edge though as it is sharper and easy to rub through. Clear would b ot really help avoid this.

Assuming your car is Not Black and only the mirror is black just paint the whole thing, it will never be compared to the other side. No blending needed then. We are now assuming you have a rattle can you are using to paint the whole thing.