I've been taught that a blue object only reflects the blue fraction of light, and all the other colors are absorbed. So what happens with a red laser?
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Your teacher was oversimplifying.
A surface looks blue because it reflects more light in the blue portions of the spectrum than in other portions.
But less is not none. It's very rare for a surface to be completely non-reflective.
Your laser pointer is much brighter than the ambient light. So even if most of the light is absorbed you still get enough reflected to produce a red dot.
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$\begingroup$ How many intensity does the ambient light have? Let's suppose it's X. If I had a laser pointer with X intensity, then in a dark room I point it at a blue surface, I won't see nothing? $\endgroup$ Commented Dec 14, 2016 at 22:30