When you first set up your blog a domain namemight seem like a unnecessary cost and if your blog is only meant to be a personal blog red by your friends and family than that might be true. If you however have higher ambitions for your blog then you should always pay the 10 bucks it will cost to get a .com domain. In some cases a .org domain might be okay as well. .net is generally well regarded but we think you should try to avoid it and stick to .com if possible. .org is suitable if your blogs that follows an environmental or social movement.
It is true that you can add a domain name later on if you blog takes of but the cost of a domain is so small and the looses from adding it later can be so great that it is always best to get it right away. When you change URL to your new domain you can redirect the old URL to the new one using a 301 redirect but you might still loose some of the value of the links that was built to the earlier URL and this slight difference might mean a drop in the serps and lower income. You can regain lost ground by continuing promotions but it will mean that you will need to do more work than you otherwise would have to.
Having a domain name also gives a more serious impression and make it easier to promote your blog. The fact that you have spent the money on a domain name make it seem like you are a bit more serious with the blog and that you have serious plans for it. Many directories will as an example not accept free blogs without a domain name attached to them. There will in other words be fewer ways to promote your blog available to you. Having a domain name will also make it easier to get to exchange blog rolls links with other blogs.
The domain name is your identity, your brand online. Having to change identity or brand later on might as you can understand make your blog hard to find for some of your readers. Even if you redirect the old URL to the new it can be hard to find you blog for readers that do not remember your old or new URL.
Regardless of whether your blog is a success or a failure you might want to sell it at some point. To cash out and get some cash for your work while allowing someone else to continue your work down the line. A domain name will increase the value of your blog regardless of whether the blog is a success or a failure. The added value by a domain name will be higher than the cost of a domain. Even a brand new blog will be worth more with a domain name but the difference in value will become greater the more popular your blog becomes. If you have a popular blog a domain name might get someone to pay several thousand more than they would if the blog didn’t have a domain name.
The single most important reason to get your domain name right from the start is however that if you do not you might find the domain name already is registered by someone else when you decide you want to get a domain name. This is especially true if your blog becomes popular as people might register the domain name simply to benefit from the reputation of your blog.