I used to be into the hoodie-under-blazer look, but now I think I'm realizing it might be kinda goofy. I think it was mainly because I felt a bit too dressed up with just the blazer, so the hoodie helped dress it down a bit. I still kind of like the contrast you can get from a hoodie/blazer combo though. Here's a cashmere one I wore on rare occasions with a grey blazer

I'll probably get some choice words on that fit, but hey it's all part of the learning process..
I also like the hoodie under leather jacket look.. can take the edge off the leather a bit. I used to wear my grey wool hoodie with a leather jacket, good for warmth and I think the contrast in texture can be nice.