That comp was in the Edit page, with a red dot render on top of a blue background render. (or shape fill from the Fusion page in place of the dot render, same result). Input color space for the shape in Fusion was Auto, changing this parameter doesn't seem to make a difference.
Interestingly if I do everything from the Fusion page (inc background) it gets rid of the black edge but the edge blend colors are wrong (See pic)... Imported renders or a Fusion shape fill have the same issue:
Interestingly with ACES color science I can only get a correct sRGB comp from the Edit page and not Fusion, regardless of color space settings. Not a real concern - it would be preferable to get ACEScg images working.
My node network is just a default Merge node with blue to BG and red dot to FG, is there some other node or setting I could try?