Even in the case of the Ron's kittens post, Pansy is the first to say black, and then Ron responds with blue (eyes). In Sally Anne's case, I think she crossed out black because she didn't want to accidentally invoke the code.

So I am starting to think the entire code is that one person says black, and the other acknowledges that they've got the message by saying blue. But what does it signify? It would probably have to be something fairly simple, like that they need to meet, or that they have left a message in some secret message place, or something else where most of the content was conveyed off-journal and the code was just a signal to look for it.

The black and blue thing is also like bruises, so it could have to do with physical harm coming to someone... but I can't make that connect to anything so for now I'm thinking that's probably not it.