Sharing Authorship of your Blog

If you wish to have another person be a co-writer of your blog it is easy to do.

Go into the settings for your blog.
Click on Settings > Basic > Permissions > Add author
Type in the person's email address or choose from your contacts.
An email will be sent to them letting them know you've added them.
Once they've accepted, they will be able to post to your blog.  You can also make them an Administrator of the blog if you choose which would give them access to making changes to the blog layout, etc.