Well, fist of all, is your existing tint a film or is it right in the glass? If it is a film, places like Zeibart will be able to remove your old film and install darker tint. I am sure you can get it in blue, as I have seen red, silver and gold tints. It is wroth a phone call anyway. If the glass itself is blue, like the top of many widshields, then no, any tint put on top will pretty much cover up the old tint and show the new stuff. Another thing to consider is that it will not take really dark tinting to make it 'really' dark if done this way, because the 2 tints effectively double up.