After some time you can attempt to make peace with them, atleast I was able to.

I had actually come across the blood season a bit earlier and was quite surprised with the effect.

I was more asking how the mechanics that make the event function work than whether or not you can make peace eventually.  

Is it war immediately, but no lingering modifier?  Or the high (+3 I saw) modifier that causes war?  Does that modifier stay permanently or fade after a specific amount of turns?  Was the "I love war" modifier even part of the event?

All the other events I've come across its been easy to tell how they functioned within the game rules as soon as they happened or within a turn or two.  This one I'm not sure about.