The blocks that are pre-added (ores like Diamond or Emerald) will work until you remove them from the list.
Trying to add blocks to the list does absolutely nothing.
Checked config, the blocks are set to "=true" but do not show up in the list in-game or get highlighted when xray is enabled.

I see that there are similar issues posted back in 2018 that seem to have been resolved, however my solution does not seem to be that easy.

EDIT: Minecraft 1.12.2, probably should have mentioned that

GIF

This looks like a super old version. What version are you using?

OH, you replied while i was editing my original text lmao
Minecraft1.12.2, Xray version 1.4.1

You're on an old version. For 1.12 we went up to 1.6.0 https://github.com/MichaelHillcox/XRay-Mod/releases/tag/1.12.2-v1.6.0

That makes sense. Whatever link i clicked took me straight to 1.4.1.
Thanks!