During the initial console setup, you have the option to enable some basic TV controls on the tablet. How does the Wii U "talk" to the TV when it's connected over HDMI? Is the TV controls sent over HDMI-CEC or does the tablet use an IR-blaster to send the signal?
It uses IR. The top of the GamePad has an IR transceiver to control the TV (I assume it also reads where you're pointing at using the sensor bar, the same way the Wii Remotes work).
In this picture, you can see the black transparent cover of the IR port on the top of the GamePad
Also, from the video on this page:
"[...] [the IR transceiver] can be used to control your TV and communicate with other devices [...]