Indeed there are no colour couplers in b/w film, but we can take those tonal relationships and use them as the elementary makeup of a colour photograph. For example, if I took a b/w picture with a red filter, the film (assuming it's pan film) would therefore only capture the intensities of the colour red in the given scene (that's why the sky comes out black, because the sky is almost cyan - the opposite of red).
The sky is, as we all know, blue. Not (almost) cyan.
And blue is indeed at the other end of the spectrum, and not let through by a (good) red filter.