Blue can be a colour; blue can be a mood; blue can be a note; indeed, blue can be different things to different people. Blue happens to be my favourite colour.
Think creatively, starting off with "blue" as a departure point for your thoughts and take a picture (or three) with the theme of "blue".
Your photo need not actually have any blue in it, but it should then still evoke "blue" in the mind of a viewer.
In order to avoid that all the entries are titled, "blue bird", "blue berry", "blue paint", "blue house", etc. perhaps we can simply leave off the "blue" bit in the titles, unless you feel it is absolutely imperative to have this word in the title of your entry.
No need to feel "blue" about this theme. I am sure there is something here for everyone.