You currently can't overwrite the previews of default blocks. Adjusting your code and adding image: FieldsBlock, text: FieldsBlock, etc. would overwrite the previews. Not a solution of course because not everyone wants to overwrite every preview. Just FYI.
Not exactly an issue of this plugin, rather a core issue I think: https://forum.getkirby.com/t/default-blocks-previews-hardcoded/23612
You currently can't overwrite the previews of default blocks. Adjusting your code and adding
image: FieldsBlock
,text: FieldsBlock
, etc. would overwrite the previews. Not a solution of course because not everyone wants to overwrite every preview. Just FYI.