I can't speak for the particulars in your case, as I have not seen it, but generally speaking what you are describing can occur after "n" number of failed authentication attempts. This is a security feature designed to prevent brute force attacks.
It is not "typical" that the support representative would not know how your usernames got blacklisted, but it is typical that if it was a security related issue he/she may have declined to discuss the particulars of the issue with you until the matter had been looked into by members of our abuse/security team.
Often, the matter will be handled by these team members exclusively.
If this is/was the case, it may take longer for you to get a response as there are fewer people working on this team than are working on the general technical support team. While DreamHost endeavors to answer all support request within 24 hours, and is usually much faster that that, some things will need special attention and may take longer to address.
The answers to your final questions are that "your responses are sent via email" and "you submit a support request".
As email is not perfect, you should check your support history (Support -> Support History) in the web panel to see if there is a response there that you did not receive (spam filer, failed MTA, etc.). All support contact regarding your account is archived there.
Support personnel will investigate every support request received. The amount of time it takes for them to respond to you depends on many things including, but not limited to, the number of pending support requests at any given time, the nature of your issue, and what research needs to be done to properly address it (as discussed briefly above).
If the standard goal of a response within 24 hours is not acceptable to you, you might consider adding the "call-back" option to your hosting plan.
--DreamHost Tech Support