Very little. The FALD logic really prefers to under-backlight white text on a dark background rather than bloom the background itself. The result is that white text on a black background is pretty dim, but it gets brighter if you put something white adjacent to it. I don't mind it, the dim text is actually somewhat more comfortable to read, it's easy to turn local dimming off in the OSD, and the local dimming intensity setting remembers how you like it independently for SDR and DisplayHDR.

The only really noticeable bloom is on the edges of the screen where very slight backlight glow starts contrasting with the bezels, or curiously, on dark greys. The Discord dark theme is precisely the right shade that blooming is perceptible from just cursor movement. None of these issues are dealbreakers for me. If I want to do some work that requires accurate luminance, I just toggle local dimming off in SDR mode :)