So I have one site, which is my main site that I have my domain set to use it. Netlify is handling the DNS for this domain. So I also use Netlify to play with some things. Is it possible to have my blog site go to mydomain.com but then have another site be allowed to be mytestapp.mydomain.com?
sure it is! its absolutely possible on all plans. what you are referring to is a subdomain.
Here is some more info from our docs:
@Perry Do I add a new domain and put it as mytestapp.mydomain.com or is there a different place?
Hey @jrock2004,
If you’re using Netlify DNS, adding mytestapp.mydomain.com to a different site should just work. Please let us know if that’s not the case (and share your Netlify url so we can take a look 
)
Thanks @jen I got it working.
Awesome, glad to hear 