NOTHING! They’re the same thing. 
But a student recently asked this question, so I though there must be others out there wondering the same thing. A name like “Fender Blues Deluxe” or “Hohner Blues Harp” for example are both diatonic harmonicas. “Blues” is just a brand or model name.
In the world of harmonica we pretty much use “diatonic” to differentiate from a tremolo harmonica or a chromatic harmonica as I talk about in this article:
But generally speaking, if it’s got 10 holes and there’s no button on the side, then it’s a diatonic harmonica, which is the kind of harmonica used for blues (95% of the time.)
Rock on,
Luke