Hue here are a couple of things to play with to avoid the spots when smudging:

Reduce the opacity of the smudge tool - take it right down and experiment to find a good balance for clean smooth smudging.

The speed at which you move the smudge tool also increases or reduces the spot effects so try moving quicker and in short smooth movements.

Finally "fudge the smudge"? and this works really well on skin tones - use the airbrush on low opacity and small brush settings to create a blended effect by overbrushing the area you want to blend - several people do this really well but take a look at some of Selinadie's portraits to see the technique and the results.

Hope these help

thank you..But,I just mix a few colors then flatten using smudge tool..you can use a photo as reference .