Im using starlight-forge-1.0.0-RC1-1.16.5.jar

When breaking blocks this black blocks would appear for half a sec, it's really weird and kinda annoying.

No timely response

Most likely a mod conflict, can you provide a modlist for the devs?

I tested locally, by recording and inspecting frame-by-frame, and could not produce this issue. Could you provide a mod list, and perhaps test other blocks (like grass)?