If you get reblogged or posted at in a post like this, its a scam.
This is a phishing attempt to try to steal your account, it uses a fake log in/verification page to try to emulate Tumblr, then once you log in the people running the fake site get your account info. best thing to do is block/report and move on. To report then block remember to go to the 3 dots on the profile, then go to the whistle and report _[insert name here]_ section, mark it as spam or impersonation, then after go back to the 3 dots and block them.
Remember Tumblr will never try to access you by posting at you. Usually they access you by email. Also the official Tumblr accounts have a specific badge verifying it's them. This badge is a blue shield with the Tumblr T in it. Also none of the official Tumblr accounts are just numbers or letters.
And I will charge my phone i swear, and I don't know if how I stated is exactly how it works, I just know how past scams like this usually operated