This requires a Tumblr account.
Take the name of the blog and add it to the end of this URL: https://www.tumblr.com/dashboard/blog/
Now the blog will open in a side-bar window on your dash with the warning "This Tumblr may contain sensitive media." and a button labelled "view this tumblr" that allows you to view previously blocked blogs.
However, I have found no way to get around individual posts being set to private, the blog owner can still view them but they cannot be made public again.
Post detailing how artists can access their own content and including what I've outlined above: https://jackthewolfen.tumblr.com/post/181375197306/jackthewolfen-jackthewolfen-gene-tierney
Please, spread this around so people can continue saving art before it's deleted or set to private!