Hi there,
First, if i want to make a part of my image transparent, i use to add (Alpha channel) using Gimp software.
Now after that, you have many options to fetch for black color, i suggest you write a black color filter. next, you delete the fetched result. and that should do it.
if you have problems with Emgu coding, i suggest you go to
www.emgu.com/forum
Good Luck,
z3ngew