I was using Volto 12.5.0 and following https://training.plone.org/5/voltohandson/introtoblocks.html
Now while creating new block ie custom block suggested approach was while registering in config
export const blocks = { ...defaultBlocks, blocksConfig: { ...defaultBlocks.blocksConfig, ...customBlocks }, };
but this did not cause any changes so I had to directly change defaultBlocks object
defaultBlocks.blocksConfig = { ...defaultBlocks.blocksConfig, ...customBlocks };
This made the new block available.
Need help in understanding why the default exported blocks object is not causing any changes, or am I doing something wrong.