Try charging via a pc, or with an original (not chinese) cable and a fast charger adapter with more than 1 amp
Is it flashing slowly or quickly? Quick flashing means there was a battery charging error.
Possibly a faulty battery, but since you've only had it for a couple of days you should return it to the store and get it replaced right away, if you bought it in a physical store they will hopefully let you exchange it for a new one on the spot.The blue light starts off flashing slow but then after an hour approx it starts flashing fast.
I managed to solve the problem with the fast blinking blue light when charging by trying a different usb cable that I plugged into my iPhone charger plug.I'm having the same issue. Battery charges, but goes to fast blinking blue when it seems to be done. Charge looks good so I'm not sure it's a problem. I've got another battery on the way I'll see if it behaves the same way.
Just needs to reset the Tello's Wi-Fi segtings, so turn the Tello on, then press and hold Power button by 8 seconds untill Tello restarts, keep in mind, thos will delete all Wi-Fi settings, including Wi-Fi passwoss