This came up in a game last night. We drew the Planetary Alignment mythos card, which is a mystic environment that gives all spells a Sanity cost of 0. However, the Miriam Beecher Blight card was in play, which increases the Sanity cost of all spells by 1. The question is, in what order do these cards take effect? If the Blight card takes effect first, the Sanity cost for all spells is increased by one. Then the mythos card reduces the Sanity cost of all spells to zero. If the mythos card takes effect first, the Sanity cost is reduced to zero before the Blight card increases the Sanity cost to one. Therefore, depending on how you look at it, the Sanity cost for spells is either zero or one.
The KiY rules say that "the effects of Mythos cards are unaffected by Blight cards." However, I think that's just in reference to the rule that you lose 1 Sanity/Stamina if you encounter a blighted character. I suppose I could go with the old standby: "whichever interpretation is worse for the investigators", but I was wondering if there was something more tangible.