If you try another cable and that is not the problem You may have to open up the back of the monitor and first try turning the Blue guns colour adjustment. Sometimes these can be reached through some service holes in the back of the monitor without taking the back off. (these are located in an easy location to reach usualy on the back of the guns circuit board) Lables "R" "G" "B" if this makes no difference then it may be that the gun itself is bad or a solder joint has let go on the Blue circuit.
Do adjustments with caution as there are very high voltages inside the monitor and remember where the adjustment pot was before you turned it so you can turn it back if need be.
EDIT: Ok I guess I missed that you did try a 1960 and it did the same thing. If this is the case then I would make sure that your pins on the Amiga video out are straight and none are missing or dammaged. check the circiut conection from the MB to the RGB out and maybe resolder them carefuly.
