Following https://jira.mariadb.org/browse/MDEV-31743?focusedCommentId=266105&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-266105:

We could store pointers to blob keys separately in a list and modify the compare of rows to
also check the list. This would allow us to do distinct for blobs of any size.

In other words, modify the code here:
if (compare_record(table, first_field) == 0 &&
(!keylength ||
memcmp(key_buffer, key_buffer2, keylength) == 0))
->
if (compare_record(table, first_field) == 0 &&
(!keylength ||
memcmp(key_buffer, key_buffer2, keylength) == 0 &&
compare_all_blobs() == 0)