You can keep your domain :)
Blogger is not a domain registrar. When you signed up, you should have received an e-mail from Google Apps detailing whether the domain was registered through eNom or GoDaddy (the registrar partners used for domain registrations).
If you don't not have the domain purchase confirmation e-mail, the DNS settings may be completed by observing the following process:
Log in to Google Apps using Blogger domain name, username, and password
Go to Domain Settings > Domain Names > Advanced DNS Settings to find the eNom/GoDaddy account information for the domain
Log in at eNom or GoDaddy to complete any desired changes
Not tested: Instructions culled from Blogger Help and The Blog Doctor (oddly enough, this process does not appear to be described in detail at the Blogger Help site).