https://mariadb.com/kb/en/set-data-type/ says:

must be chosen from the list of values 'value1', 'value2', ...

I tried exactly that using the following DDL:

alter table testtabelle add column Blutgruppe set('a', 'b', 'a,b', '\'') not null default 'a,b';

I got this:

[22007][1367] (conn=41) Illegal set 'a,b' value found during parsing

To avoid uncertainties like these, the valid values for a SET data type should be documented.

As far as I could see in Column_definition::create_interval_from_interval_list, only the comma is an invalid character in the values of the set. It should be clearly documented that this is the only character that is not allowed and that this is the only way to avoid ambiguities in the default clause. Is that the only reason why the comma is illegal?