After reading the whole poem here is what I understood ( I understood this poem as a man writing about a woman).
I don’t love you as if you were a rose of salt, topaz,
He began by saying He doesn't love her because she is pretty to the eyes.
I love you as certain dark things are to be loved,
When he says dark things,he might have meant that she is ...
I'd really like to see the full text of your translation. It differs from the two translations I posted in the body of your question. In the latter cases, they use the English "obscured", but yours uses the literal translation of "darkness".
Looking at the etymology of obscure, the direct use of the English term, as opposed to the literal translation is ...
Well Neruda was Chilean and Chile is in the southern hemisphere. Therefore January is the hottest month by far and when the sun shines more cruelly.
I am not Chilean but I am from the same latitude in Argentina. As January is the hottest month it is usually the month with the brightest light also.