I have a client that would like to create an email blast to 50,000 or so constituents, and we are currently evaluating CiviCRM to handle this. I've noticed the documentation says that CiviMail is good for email blasts sent to over 50 constituents, but I can't find any mention of whether or not there is an upper limit.
Absolutely - that can be done - the 50 is just a reference to the Search -> Find Contact screen -> Search -> Select -> Actions -> EMail send now (to 50 or less) option.
When sending out lots of CiviMail to lots of CiviContacts best to use an external SMTP service to help you with the Email delivery - and to minimize the risk that your webserver will be blacklisted.
UPDATE an actual number from one of my clients' civicrm tables -> intended recipients: 89419 And I'm sure there were larger mailings.