Hi Guido,
Yes I use the same technique you do.  I don't worry about what the
background looks like when I'm taking the picture or doing the scan.
I concentrate fully on what the shell looks like.  Then, once the
image is aquired, I use Photoshop to select the background, and
remove all color, contrast, and brightness from it, thereby producing
a uniform black.  Also, I like black because it looks good with any
color shell.  A blue background looks good with shells that have a
lot of warm hues, but not as good with shells that are greenish or
violet in color.  Black can be difficult though, when photographing a
shell that is largely black, or a very dark brown.  Then I switch to a
lighter background.
Of course I realize that photographing shells on film is a different
story - then you have to be concerned about subject and background
simultaneously.
Paul M.