Good point.
But there are common charecteristics that must be present for something to be considered a certian style. Just becuase the blues scale was used in writing the melody, or dominant 7th chords used to create the harmony doesn't make something a blues song, but it may give the song a bluesy feel.
One of the most common characteristics of the blues form is that it is always divisible by 2 or 4 (phrases and form). So all blues (of the traditional sense) follows a 8, 12, or 16 bar form. As far as new forms or progressions being written, yes it does happen. What makes a "new" progression "Blues"? I guess it's popular embrace by the musicians of the blues community. For instance, Stormy Monday was a popular song that was written in the late 50's early 60's that was embraced by the blues community. A lot of blues musicians took the chords and form of that song and created there own. Same with "Night Life" by Willie Nelson, and "Key to the Highway" by Eric Clapton, again 2 songs that were embraced into the blues community.
It's hard to explain 'what" characteristics have to be present for something to be labeled a certian genre. One of the biggies we seen in the past 15 yrs is the existence of "Smooth Jazz". Although smooth jazz utilizes some characteristics of jazz music, not nearly enough to be considered a Jazz art form. In reality its more closely related to rock and funk. There's no way you can seriously compare Kenny G or Gerald Albright to John Coltrane.
The same argument you brought up could be presented against classical forms such as sonato, rondo, fugue, minuet, etc.