Blu-Tack doesn't harden, it stays sticky. It's used to attach posters to the wall and stuff like that. I wouldn't want to use it on the edge of a MacBook, it would just gather dust and grime, and would easily fall off.
Solution is to apply some Sellotape (Scotch tape) to the back of the poster first, then put the Blu-Tack on the plastic tape, which blocks the solvent. And avoid using it on wallpaper (emulsion or gloss paint is fine). Better still, use poster holders and a picture hook, or go the whole hog and frame your artwork properly.
Sugru ... air-cures: it doesn't have the solvent-leaching problem, but if you used it on a poster the only way to get the poster down would involve a chisel or sandpaper. Blu-Tak is for temporary attachments; Sugru is intended to be permanent.