There are a number of differences between these two small birds. Firstly the Great tit has a black stripe going down its breast to its tail and the Blue tit only has a small black incomplete stripe. The great tit also has a black cap and the Blue tit has a blue cap. The Blue tit also has more white on its face than the great tit. And a subtle difference is the Great tit is slightly larger than the Blue tit.
The great tit is also different from the Blue tit.