Happy to see you here. I am very excited to have you along with the community.
Firstly, even WordPress is not friendly when coming to coupon sites. If you talk about blogger, the scenario is even worse.
You can build an excellent blog about coupons on blog spot and rank. But you cannot expect it to turn out to be a brand in a long run if you continue with Blogger or for that sake WordPress as well.
I have worked on about four coupons sites. 3 of them were on WordPress, and all of them failed. Only one was a custom platform, and it's doing pretty well.
I will do research on good themes for coupon sites for Blogspot, but it's recommended not to choose blogger for coupon sites.