A sudoku with a surprise

Here is the grid for filling up by you-should not be difficult-The cells which were left blank have been marked b-
bbbbb96bb
6b5b8bbb2
bbbb4bbb9
b6185bb7b
b5bbbbb8b
b8bb1456b
5bbb9bbbb
8bbb7b1b6
bb73bbbbb
Fill it up and find out what the surprise is. I cannot reveal it now as you will be able to fill up the grid more easily.