If I read this correctly, all you need to do on your event window timeline (once you have enabled animation) is to create keyframe where you want the effect to commence and set the blend amount to what you want. Then move your cursor to where on the timeline you want the effect to finish, and move the blend amount slider to where you want it (probably at the opposite end of the slider to the first keyframe). You can experiment too with rightclicking the keyframe and selecting any one of the properties listed although this won’t make much difference to this particular effect.
Too easy so I may have misread your issue!