To be clear, a blues progression is called a I7 IV7 V7 progression rather than I IV V which implies diatonic chords (in Bb that would be Bb major, Eb major, F7).
Blues progressions actually do not conform to standard western musical harmony, you are correct by saying that improvisations will sound strange when playing diatonic major modes over the equivalent of chords.
Traditionally, blues progressions are almost always improvised over using a minor pentatonic or blues scale of the key that the blues is in. They are often very straight forward harmonically.
My personal approach to improvising over blues scales is to rely heavily on the key's minor pentatonic and blues scale, while adding in chord tones from the IV7 and V7 chord where necessary rather than using the other extensions of the scales that you would normally play over those chords (5th mode harmonic minor, etc) which can sound a little strange because they sound like they take you into other keys (bVII major over the IV7 chord etc).
If you are going down a slightly jazzier path, you can implement whole half and half whole diminished scales over the chords and as long as you end up resolving onto a chord tone of the current chord (I III V or VII normally, often the major 9th as well) you can get a pretty good sound.
Another thing to note is that you can basically play anything you want to on a blues as long as you take the above note into consideration, end up in the right place and play it with feeling. After all, blues songs are all about the feeling (not the complication of harmony).