Just double click on it and you will be presented with a Red, Green and Blue entry boxes. Make red and green zero and make blue any number from 0.5 to 1.0.
Not sure what you mean there. When I double click in NifSkope, I don't get the Red, Green & Blue entry boxes, or anything at all for that matter. When I click on the icon next to NiMaterialProperty, I'm presented with options to change Specular Color, Alpha & Emmisive Color, but changing these has no effect on the appearance in the GECK. Obviously I'm doing something wrong here, but what? If you want to look at the files to find where I borked up, here's a link: http://www.4shared.com/file/SA0YdDAk/Problem_Map.html
Grr..Simple retextures like this are sooo much easier in Oblivion.