If you have a hobby shop nearby, see if it carries an acrylic paint by Tamiya. Colour code is 'Flat Black XF-1'. I am sure that there are other makes. Acrylic model paint is easy to clean up with 99% Isoproyl alcohol even after it has fully dried. It's the same alchohol used for cleaning lenses so is OK to use. I would use a small camel hair artist paint brush and try to paint the actual chipped area if possible, depending upon which side of the lens is chipped and maybe 1/2mm beyond onto the undamaged glass. One coat should be enough. If it doesn't work, you can just remove the paint with the alcohol and lens tissue. I have used this method for blackening the edges of lenses etc and even to paint out a 5mm band around the edge of a reflector mirror to 'cover up' a turned edge. You will be able to see the painted area in the out of focus positions but not when in focus. The main thing is to remove any glints, scattering and defects in the image by preventing the light from passing through the chipped area.
I found that most marking pens are too transparent and do not cover the shiny edges very well.