Blending on base/clear coat vehs is basically the idea of taking the panel that is being refinished, and extending it in to the panel next to it (ie rear deck lid, blending in to the tops of both quarter panels), so that your eye cannot see a line of difference in color.
the shop will first tint the paint to match the existing color as close as possible. this can take a few attempts on a sample until the color is a match. this is only necessary on older veh's which have experienced paint fade, as brand new cars should be fairly close to original color.
the adjoining panels (quarter panel for rear deck lid, and fenders for hood) are taken down to the color. the hood and deck lid are sprayed with color, and the color is "blended" or sprayed on to the adjoining panel. this "fools" your eye so that you do not see any true break in color difference. both panels are then clear coated.
elementry explanation from a tech side, but hope this helps
1986 Volvo 240DL - 100k
2002 GMC Envoy - 19k
Sold my Cummins, I've got the fever for a Benz Diesel!