You can use tags for this. For example, tag all your plant entries plantlife, street art streetart. You can give one blip multiple tags, just enter them in the tag box separated by commas when you upload your photo (if you want to add tags to existing entries, you can do so by editing the entry).
Then you can use the search box at top right to find them, like so:
Entering #plantlife in the search box will find all blips tagged plantlife by anyone.
Entering #plantlife @me will find all the blips tagged plantlife by you.
Entering #plantlife @blippername will find anything tagged plantlife by the blipper with that name.
The only rule for tags is that they can't have spaces in them. I'd advise not putting punctuation such as ' or & in them either, since Blipfoto doesn't seem to cope with that very well when you search. You can see examples of how tags are used on this page: