You should be okay
Tumblr defaults to HTTPS. As long as you didn't install the company's Malcolm in the Middle certificate (if they have one) on your cell phone, then you should be fine. They'll know you connected to tumblr, but won't see exactly what you visited, you degenerate.
But if you get "caught," just be honest
If I'm somehow wrong, or you installed their MITM certificate, and you are confronted about this in the future, you should simply tell the IT/security team the truth, assuming your current description is true.
But if you get "caught," just be honest
Most places value honesty above most things. I've seen people get away with visiting an adult website simply because they told the truth. "I activated private browsing, and muscle memory defaulted to my usual adult website. I am SO sorry." It was a one-off freak incident that never happened before, or again, and the guy closed the tab immediately while /facedesking. We even caught his /facedesk on camera.
Having said that, I recommend everyone stops connecting their devices to corporate WiFi/networks, and this will never happen again. I would not bring my own devices onto a company's corporate network, ever. But I'm paranoid, so there's that.