Let me put you on my number one secret to perfecting the blend.. Just remove enough hair. I know we're afraid to take too much off because once its gone theirs no going back but, thats how you get it nice and tight.
Here are some of my tips:
#1 Remove bulk of hair before you put your first guard on a clipper.
#2 Blend tight ( make sure all hairs are going the same way)
#3 Control & Consistency throughout your haircut
#4 Go with the tempo of the your clipper (let it guide you)
#5 Always take pictures of your hair cuts (then compare as you practice)
Let me know how you perfect your blends.