Hi,
don't use BLANK() - it can produce wrong result.
To check 'nonemptiness', just use IF({Field}, ...
Also 'value_if_false' can be omitted in IF, default is empty string.
So, just type IF({LINK}, *your formula* ). Brackets can be omitted for one-word field names
IF(LINK,
SUBSTITUTE(', '&LINK, ', ', ', https://example.com/')
)