Your way would work, but I think it would be confusing to keep track of the offsets and the sizes of each section of the bitmap.

With my way, you'd have :

 A       C

333     444
222
111

C->B  B   A->B
     
     444  444
          333
          222

And every time you needed a new blot you would create it on C, draw it to the unused bitmap, and then draw the rest of the connecting blot from the bitmap in use, and they would swap places. I suppose you could do away with C and draw the new blot strip directly on the unused bitmap.

That's the general idea, yes. The old blot shifts down and the new blot takes its place at the top. It just uses two bitmaps instead of one to avoid drawing onto itself.