I have seen couple of sites which has been explaining to the user how to take blog backup. But I am surprised to see that most of the them tutorials are based on classic blogspot interface. The tutorial will teach you how to take blog backup in the modern blogspot interface.
The process is very simple. First of all, you have to log in your blogspot account. After logging in, you have to chose the blog, of which you want to take the backup. After making the selection, on the left hand side you will see like this.
At this point, you have to go for settings. Next, you will see something like this.
Now, you have to choose Other option, then you will get a window like this.
Now, here you can see 3 options. Just select the Export Blog option and you will get the window like this.
Just hit download and save your blog backup in XML format.