I tried searching for this issue but couldn't find a duplicate.
Effectively, the block cursor has been practically illegible for the game's existence, and especially with high density displays, this issue is only getting worse.
The block cursor is always black with lines one pixel or two pixels large-- not Minecraft pixels (1/16m), but pixels on the physical screen. This is especially difficult to read on dark blocks, but in general, you have to squint to see it, especially when counting blocks.
I've attached some images of the block cursor, one on obsidian, and one on dirt next to some grass. The outline on the dirt is faint, and the outline on obsidian is effectively invisible.
Having either more contrast on the cursor and/or more thickness feels necessary to ensure it's visible.
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I'm explicitly filing this as an accessibility bug, not a feature request, because the issue is that the UI element is illegible.

@unknown Yeah, I just wanted to mention that the last part seems like a feature request. But overall this report is valid 😃
Please attach a screenshot or video showing the issue, and steps to reproduce if applicable

This isn't resolved, but okay, I attached some screenshots.
I hadn't attached some initially because like I said, this has existed in the game for a very long time. However, I guess it's fair to want to document how things currently look in case they get changed in a future version.
Related to MC-210369
Can confirm in 1.21 Pre-Release 3
It should have a configurable thickness as an accessibility option and change colour to be more visible against darker blocks. Both can be gated as accessibility features, but I think they should exist.
This part could be a feature request.