If you are being stalked and are receiving death threats and hate mail or email you should contact your local police department and file a report. You can print these threats as evidence. If these threats are coming as email you could check the full headers of the email and then report them to the ISP they came from. If they are leaving these threats as comments you should report them to Tumblr -
https://www.tumblr.com/abuse
Here is the Tumblr help page -
https://www.tumblr.com/help
If you need to know how to block someone you can type "Block" in the search box.