Hello Sandro,
Sorry to be late in responding. Have been away and took a break from all things digital.
About 'only blue channel': the foveon sensor, used only in Sigma cameras, is a 3 part sensor, arranged in a a stack of three layers. The top layer is blue, the other two are red and green. By choosing to use the blue information only you get exceptionally clean, sharp, and high resolution b&w images.
And the way to get just the blue layer is to utilize the color wheel in the Sigma software program and drag the center point all the way to blue- with no green or red. You'll find it's easy to do. If I can do it, anyone can!
Be glad to offer any more help.
Glad you like this foto.
Richard
PS- I use a Sigma DP2 Quattro and the latest version of SPP.