I had a similar problem...that is the reason for posting my question...I have been using Marshall's Spot All - neutral black....and I did everything exactly how it was suppose to be done..the color was even lighter than the tone in the print, to avoid making darker spots...and then it changed color as soon as it was put on the print.
and from my experience...it is not invisible when it drys..even when its the blackest black and the colors should match...to my eyes at least.