I tested this ten times using Red Pokeblocks (Which are spicy). The Natures of the Pokemon I caught were Docile, Adamant, Lonely, Modest, Lonely, Lonely, Bold, Mild, Serious, and Brave. Pokemon that are Adamant, Brave, Naughty, or Lonely like spicy Pokeblocks, and Pokemon that are Modest, Timid, Calm, or Bold dislike spicy Pokeblocks. Five out of the ten Pokemon like spicy Pokeblocks, and two out of the ten Pokemon disliked spicy Pokeblocks.
I'm not adding this as an answer, as I'm not completely sure about how the Pokeblock feeders effect the natures of the Pokemon you encounter. (For example, I'm not sure if none of the Pokemon I caught were supposed to have a nature that dislikes spicy Pokeblocks, or if all of the Pokemon I caught were supposed to have a nature that likes spicy Pokeblocks.) If you would like me to add this as an answer, I will.