Sounds like a disaster.

Admit it, you want to wear your new blazer but you are afraid people might ridicule you for being "too formal". So you want to try to dress it down with the t-shirt. Right? Am I right?

Instead of going for something totally casual like a t-shirt (which totally doesn't go with slacks and/or jacket), you can dress it down using cloth material and colour.
Like instead of a dress shirt, go for a oxford cloth button down.
Instead of a pair of trousers, go for coloured chinos.

The material of the blazer is also important. Since it's from Topman, I'm guessing polyester blend?

By the way, the black slacks is fine (you are talking about those polyester "smart" trousers right?) if you are selling insurance or MLM.