I think the problem here, although I'm no expert with Retroarch, is that "input_player1_b" is the "target" key, whereas "1" is the source key. So in this case you're binding the number 1 ( which Picade doesn't output at all ) to button b.
Honestly, I don't know if Retroarch supports binding keys like this but we've got a few people here who, I think, should know :D