Good question, since the majority of paint repairs are blends. We will use acrylic enamel for this example (others are similar). After you have repaired the area (body filler, polyester putty, primed, etc.), you must clean the surrounding area with soap and water and a good scrubbing with a final wash solvent (mild, silicone free). I cannot stress the importance of this step enough. It is the singular most important factor in your blend. Clean to the edges of the panel (where the masking starts). After the area is sufficiently cleaned, follow with a thorough compounding of the area (my personal preference is #4 or #2 McGuiars and a wool pad). This will remove old oxidation, and disturb the surface enough to promote adhesion. Clean off any polishing residue.
Reduce the air pressure to the gun to around 15-20 psi (siphon feed), and spray the repaired area. Wait for the coat to tack sufficiently, then apply successive coats until full coverage is achieved, slightly extending the coats each time to melt in any dry spray from the previous coat. On the last (blend) coat, reduce the paint in the gun (you may have to pour some out) 100% with blending solvent. Carefully blend out the dry areas. Further addition of blending solvent may be necessary, or if you are very careful, you can lightly blend the edges with 100% blending solvent. ** Note** If you don't have blending solvent, you can use medium reducer, although it doesn't work as well. Don't use fast dry or slow dry solvents, as they contain additives which are not conducive to good blending.