RSMB do not blur black text. I have to make the text white, add RSMB and then add Tint to map white back to black. This workaround works, but is there a way to blur black text with RSMB without this workaround?
In practice what happens internally is that on a setup as you describe you end up with black text on black which RSMB cannot deal with.
To sandwich it like this:
Also, for your information, with flat (no texture) large text, it might work better if the text motion is the same for all characters to use RSMB PRO without image tracking and use the Tracking Points in RSMB PRO to indicate the motion OR simply use SmoothKit Frame Accumulate then RSMB if the each character is animated separately.