Last time I looked, there wasn't an iPhone app for blogspot blogs. You could, of course, use its web browser, but that's a little slow fir the purpose if you don't have a 3G or wifi connection.
Now, there's iBlogger, which supports a wide range of blogging engines, including blogspot. This is my first post with iBlogger, but I'm expecting it to be mangled somewhat because I need to change a setting on my blog.
It has some nice features. Allows tag management, lets you add links, and will use the iPhone's location services.
Mobile Blogging from here.