"rava would not decre fast days on cloudy days" - because of the pasuk. - does this not seem to be strange ? almost like kishuf, no? becasue of a 'drash' which does not seem to be so direct, he did not declare a fast which was needed? seems tamuha, no?
First, it is not a direct Derashah from the Pasuk. The Sifsei Chachamim (Berachos 15a) points out that the words "Mishum she'Ne'emar" or "Mishum d'Chesiv" (as opposed to "she'Ne'emar" or "d'Chesiv) mean that he is merely finding a support for his view in the verse, but not that he is deriving it from the verse.
Second, if the verse says that a cloudy day is inauspicious, that is obviously not Kishuf. If you want to know the deeper reasoning behind it, the Razei Berachah explains that Tzadikim live with the realization and understanding that nothing happens by chance, and that the outer appearance of things in the natural world indicate what is going on in the higher worlds. After the Churban, when this concept of "Sakosah b'Anan Lach" became relevant, Rava saw that it was not a good day for Divine mercy and thus refrained from decreeing a Ta'anis. (See also Yevamos 72a and Rashi there, DH Es Ratzon).