To download data from Tumblr, you should make use of the Tumblr API
Most public data on Tumblr is available over the API.
To see a list of all possible endpoints on the API, visit the reference: https://www.tumblr.com/docs/en/api/v2
The reference will also explain what information you can get out of the Blog and Tag endpoints.
To scrape data from the Tumblr API, you will need to create an app.
Start by signing in to Tumblr, navigating to https://www.tumblr.com/oauth/apps and creating an app
Once the app is created, scroll to the top of the page and copy the OAuth consumer key into Reaper: