Timestamps for BLOB fields

I'm putting together a small database to track communication with
our customers as we make some changes to our service. I want the
database to store a diary of all the email we have with each
customer on this subject, and I'm using BLOBs to store that
information.

I also want to have a timestamp for the last change date of this
email diary, so we can easily see which customers we haven't
talked to lately.

Normally I'd do this with a trigger, but I'm not sure how to do
that in this case. Once the diary is created, the oid for it
doesn't change, so I can't trigger on the diary field in the
table. The actual contents of the BLOB are in a system table and
I can't trigger on them.

Anyone have any ideas? Should I just depend on the client to
update the timestamp? That certainly wouldn't be hard, but it
feels sloppy.

Thanks for any help you can provide.