At a 50% boost the rate during rare enemy week is 1/333.33. Take 500 and divide by 1.5. If you only killed 250 Hildebears as you said in the previous post, then I don't know why you'd expect to see a Hildeblue. If you did the Forest 2 section of Tyrells's Ego 20 times as you said in the OP, then...