I don't think an air brush is going to give you enough volume and fan pattern to do the job. A small touch up gun sometimes referred to as a detail gun may would work well. Personally, I use a standard full size gravity feed gun and just turn the volume down a bit and the fan appropriately for the size of the area.
If you cannot go to a definite edge or a graphic edge and have to blend I use the "two gun method" of blending. One gun has the paint at full strength, the second gun has a 50/50 overreduced mixture to "melt" the edges of where you blend.
When it comes to the clearcoat, I use the two gun method as well but have straight reducer in the second gun to melt the edge. This keeps the "dry look" of the edge melted in which results in much less polishing afterwards to eliminate the blended area.
I know it sounds complicated but it is actually quite easy, just time consuming. The prep is hands down the most time consuming.
Mike.