Answering some comments, Sinclairean does not support hi-res raster border rendering yet, so it appears solid regardless of changing its color many times a frame.
Actually, hi-res color is not supported at all yet even on the active screen area (i.e. Shock Megademo multicolor section does not show up OK), only beam synchronized video writes are supported (the minimum required to run games like Cobra, Arkanoid, etc.)
As for the interframe blending, it is actually synced with the electron beam, so it should not show any out of sync artifact. If you come thru any artifact please report game and situation.
NB that pausing/still advancing an interframe blended moving graphic DO show "ghosts"/double images that actually build up the effect (a percentage of the current frame is mixed with the complementary percentage of the previous frame.)