That's a huge buff. Sion is plenty strong. Hes also the most passively tanky champion in the game. He doesn't need anything more. I assume your blocking 10-40-70 on autoattacks and even assuming just autoattacks that is MASSIVE buff and you probably want to block all damage. Be more specific with your ideas or you are useless. You don't say what kind of damage he blocks and you don't say weither it does it before or after resistances which is a really big difference. Also, blocking a flat damage against aoe will make it literately worthless or do around 10 damage a second if they're super fed after they go through your resistance.
Edit: Hey asshole. Don't edit post without telling people you edited the post and what you did to it when someone has already commented. Also, Your idea is even more worthless now because again, you can't just block none specific flat damage. It needs to be resistance or against certain types of damage such and aoe or autoattacks/single target spells. Right now there exist no none specific flat damage blocks for good fucking reason. Don't fucking make 3 exact copy and paste forum post.