Ok..this may sound extreme, but I would try Kilz Original. (brand name stain killer)
They have 2 versions: water based and oil based. The oil based one ("Kilz Original" has higher stain killing properties. You can buy it at home improvement store/builders supply. You can go over the sharpie marker lines (not the whole mural)..with 1 to 2 coats. Then maybe gesso over the kilz before repainting with the acryics so it doesn't soak up the acrylic pigments and dull them out..
Incorporate the outlines as part of the compostion and paint over them with a mixed black from your palette.
This is a suggestion with no sure-fire guarantee, But I have used Kilz around the house and it covers many types of stains.
Link to Kilz Website