Question

We see any object when light reflects from that and falls on our eyes. The colour of the object seen is the colour of the reflected light. When we see a black-coloured object, it absorbs all colours of light and so we see it black. But then why it is not invisible to us, since no light reaches our eyes.

Answer 1

We do not see a black object...  our brain does intelligent understanding of the presence and feel of the black objects.
We feel the absence of colors reflected as being black, or dark.

there are some impurities in the black object like dust or other particles.  So we do see reflection of light from them...