Hi,
I have found the following very helpful for getting a good blend.

When i have been for a cut in of a new piece,i find the blend is never spot on,so when i detach,and wash,dry the piece,and gently brush the hair,i pin the piece on a stryfoam head,then i go around the edge of the piece hair with a gillette razor.
I scrape gently at the ends of the hair going down in short motions here and there,this also tapers and thins out the ends of the hair so it sits flatter against your own hair,its imortant to avoid having a line in the piece hair,if its all cut the same length.The razor will clogg up but just brush the hair off the blades.
I only do this gradually (and i mean gradually,you don't want to hack of a lot,just a little bit till you get confident with the razor),then i can see how the blend looks when you next wear,then i can do more if necessary.

Regards Topcat