In your blogspot account, I think you need to add the domain name you're using (include www or whatever subdomain you choose). In your Dynadot account, you would select the Advanced Dynadot DNS setting and create a CNAME record for the subdomain and point it to the blogspot target host. The target host would probably be "ghs.google.com".
How do I set up DNS for my domain?
To set up DNS for your domain, please follow these steps:
Select "My Domains" from the left-side menu bar and click "Manage Domains" in the drop-down.
Check the box next to your domain name(s) and click on the "Action" button.
Choose "DNS Settings" from the "Action" list.
On the DNS Settings page, select the "Dynadot DNS" setting from the top drop-down menu.
Choose the "Record Type" you want to use from the drop-down menu, then enter the IP Address or Target Host of your web server in the text box.
If you have an Email Server or MX Record available for your domain email, enter the priority (otherwise known as distance) and the hostname in the subsequent text boxes.
You can also specify subdomain records in the "Subdomain Records(optional)" section the same way you do to the "Domain Record(optional)".
Press the "Save Settings" button to save your changes.
Please be aware that it might take some time for your name server changes to propagate.
There are certain steps for each record type as well, the most common are:
Note: If you have been instructed to add an "@", this simply means that the record should be entered under the "Domain Record (optional)" section. The "@" itself should not be entered.
Our Advanced Dynadot DNS also requires an IP address for just the domain. You can use 127.0.0.1 as the IP address for just the domain, which would point a visitor back to their own computer.