I regard most of the foregoing as missing the original point and I reckon that that physics teacher would agree with me, bearing in mind his broad hint about the sun.
OK, some of us speak of "black light" as UV, but even as a schoolkid I always thought that a stupid and misleading term, and that was before I even had heard of black body radiation. I am sure that the teacher was NOT speaking of anything of the type.
What is colour? Most of us seem to agree on that, not counting a bit of a red herring about whether light is visible or not. Ignoring another (subjective) red herring about our eyes and brains, it (objectively) amounts to the relative intensities of relevant frequencies of light, together with the absolute intensity of the ambient light relative to the detection apparatus.
In particular, consider "white light".
In everyday experience, and in the absence of certain classes of gross subjective distortion (such as recently having lived in green light for a few weeks on end) this amounts to black body radiation within a given spectral range (for humans, between roughly 400 and 800 nm), and intensity ranges within a very few orders of magnitude of those typical of clear-air desert sunshine.
Now, let us take our source, diffuse the light and progressively reduce its gross intensity without altering any other objective attributes.
Still white light, right?
Well yes, but keep dimming it and you will find yourself in blackness; dark if you like. It still is radiation from the same black body at the same temperature, only there is less of it. It still is there, as you can confirm with starlight cameras etc.) And long before the light has vanished you would say it is black (or grey, which is simply an intermediate degree).
Talking rubbish am I? Try harder!
Set up a white light as described hereintofore, to shine on a suitable screen. Say a nice wide circle of roughly even intensity.
Now interpose a few filters of various intensities, but all of neutral grey (otherwise known as "dark white" in colour) . They will cast shadows of which colour?
Errrr... grey, except where it is dead black. Right?
But then, the only objective difference between black, grey, and white is the intensity.
Black light
is white light of a sufficiently low range of intensities, as originally emitted by a black body of a sufficiently high temperature. such as the ... sun?
Much as any other colour effect can be seen as a range of intensities of a given frequency mix.
Yes?
Quibbling, am I?
Then try to make sense of this well-known, (brilliant, though not white) illusion, that includes white, grey, and (in some versions) black light:
https://en.wikipedia.org/wiki/Checker_shadow_illusion