You have already improvised pentascale scats over left-hand open fifths following this 12-bar blues progression :

Now, simply improvise on an "expanded" blues scale over open minor sevenths (e.g., C - B flat; F - E flat; G - F). An "expanded" blues scale consists of the blues pentascale (1, flat 3, 4, flat 5, and 5) plus a minor 7th. Accordingly, a C blues scale would have the notes, C - E flat - F - G flat - G - B flat.

In most white keys, if you place the seventh at the top of your blues scale, finger the scale by alternating the thumb with the third finger. A C blues scale where the seventh is placed at the bottom of the figure would be fingered: G - 5; G flat - 4; F - 3; E flat - 2; C - 1; B flat - 2

In the right hand, practice the blues scales beginning on several different notes. Make sure you are able to place the seventh both at the bottom and the top of each scale. Apply blues rhythms while you practice.

Add the supporting open sevenths in the left hand. Practice the open seventh progression by itself, then try adding a right hand improvisation.