When adding a Block that supports Background Color Property, you can't add a HEX Color Value, if the HEX is a colour with transparency.
For example, if you add #cf2e2e, which is a full-colour, it works.
If you, however, add #00ff001C or #d12e2e7d etc, which are all transparent HEX values, it doesn't work.
This used to work in past *at some point*, maybe 2 or 3 months ago, and stopped after some updates, but I did not have time to debug when and where it happened.
I'm aware that you can add RGB values but that's just silly because you need to add 4 numeric values, which are not even fully visible!
Check the "Failure" Screenshot for this addon issue.
UX failure! 😉
(it costs 5 minutes to add an RGB in Blocks, due to this, whereas adding a HEX would cost a second)