Render your image against the same, or similar variant depending on complexity of composite, background color as the color you are going to be compositing against. Render Over: Backgound Color.The small fringe of pixels will then be sympathetic to your final composite. In this case black? Save out as .psd. Sometimes a quick pass along the outlines of your filled pixels with the blur tool, very low value blur, after merging, will also more naturally blend the fore and back pixels together besides. Also a very slight lens blur masked with a zdepth pass as a layer mask helps to bring things together naturally. MMV according to image and desired contrast. I generally render Sinc, filter size 2 or 3 which results in a very crisp render but sometimes requires a little post blur along edges to minimize aliasing or soften edges a little.

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W10 Pro, HP Envy X360 Laptop, Intel Core i7-10510U, NVIDIA GeForce MX250, Intel UHD, 16 GB DDR4-2400 SDRAM, 1 TB PCIe NVMe M.2 SSD

Mudbox 2022, Adobe PS CC, Poser Pro 11.3, Blender 2.9, Wings3D 2.2.5

My Freestuff and Gallery at ShareCG