If you want to open blog on own domain you should buy it before.
I’m usually looking for domain names on Yahoo

http://smallbusiness.yahoo.com/domains/?s_fptrough=dm_acq_domainname_20081001

I’m not buying domain names on Yahoo last time.
If you are buying domain name first time you can do it – price for new customers on Yahoo is $9.95 but if you’ll want to renew it next year you should pay $35
This is the reason why I’m buying domain names on Godaddy.com
Price for first time buyer for year is $1.99, price for renewal is about $10, price for new .com domain for current customers is $9.99, price for transfer domain name from another registrant is only $6.99
I’m really contented with GoDaddy prices for domain names