Blue Tack (one common brand name is Blutack) is a reusable adhesive that comes in blue, of course, but also in many other colours. Although this substance does well at keeping posters and other lightweight decorations safely attached to walls without the use of nails or tacks the adhesive will leave a small stain after a while. You can usually remove this stain from washable surfaces and many painted surfaces.
Use a dull blade (plastic or metal) to scrape off any remnants of blue tack from the surface. Alternatively, you can use a fresh piece of blue tack to lift off old pieces. If this is a painted surface and the tack is pulling paint from the wall when you scrape, stop and apply citrus cleaner to the edges to soften the tack.
Apply a citrus-based stain remover to a cotton swab. Some stain removers spray on--if this is the kind you are using, place the swab on a paper towel to protect other surfaces as you spray the stain remover onto the swab. You could spray the stain remover onto the blue tack stain directly; however, if the surface is painted and you aren’t sure how the stain remover will react with the surface, the cotton swab method keeps the cleaning area confined to the blue tack stain.
Rub the treated cotton swab over the blue tack stain. Follow with a dry cotton swab to lift cleaner residue. Allow the spot to dry.
Repeat cleaning the spot with a treated cotton swab, if any stain remains. Dry the area afterward with a soft cloth.
On some painted surfaces (especially non-glossy, matt finishes), it may not be possible to remove the stain without harming the paint--in this case you may need to repaint.