Inserting multiple rows into a blob

I have some tables similar to the following (very simplified) layout:

first_table:
id int,
first_string char(4),
matches blob default NULL

second_table:
id int,
first_string char(4),
second_string char(4)

For each first_string in first_table, there are several first_strings in
second_table, each having a different second_string associated with it.
I would like to insert all the possible values of second_string into
first_table.matches as a text blob.
Is there any way that this could be done? Preferably with sql commands
only, if at all possible.
Thanks.