/buio/ is a 'matter of fact' word, it just describes the fact that there is no light, it also has a metaphorical meaning usually implying 'not having an hint' or
/tenebre/ is used where 'buio' happens to be and when the context
relates with feelings or facts such as:
-- dramatic or tragic events
Being this the association, /tenebre/ carries by itself these meaning, so it is a good
word to be used when introducing some tale intended to produce 'suspense'
in the reader.
Of course /tenebre/ carries several metaphorical meanings, like:
/le tenebre del peccato/.
A word of caution: only great masters, like Dante Alighieri,
Fedor Dovstoyesky, Bulgakov, Edgar Allan Poe,
Alfred Hitchcock, Nick Cave (his wonderful song 'Zero is also a number' is really 'tenebrosa') just to name a few, could travel into the /tenebre/ and get back to tell us with their great narrations the world what they were able to see, albeit in complete darkness.
For reasons I don't know, /tenebra/ (singular) is almost never used.