I just updated my Noctilux article with a test of the new 60mm Leica ND-filter 4-stop (16X) which is a little more blue than the B+W.
Scroll down the page a bit to get to the part.
http://www.overgaard.dk/leica-50mm-Noctilux-M-ASPH-f-095.html
I've written up a little bit about accuracy of ND filters in general (as to colors and density).