Ecobee3 Lite install has no terminal for black wire

[dborns]

I ordered an Ecobee 3 Lite and picked the self install. I started unhooking the wires on my old thermostat which is a Trane XT500C. I found that the old thermostat has a black wire going to the X2 terminal. This is a heat pump system, so that's what's confusing me.
On the OLD Trane terminals, the wires are-
Wire Terminal
Red R
Blue B
Brown Not used
Black X2
Orange O
Green G
White W
Yellow Y1
The other terminals NOT used are H, H, OT, OT, F, and Y2.

My Ecobee3 has these terminals- Rc, G, Y1, Y2, O/B, Rh, C, W1, W2, PEK

 

[dborns]

I've googled Ecobee 3 Lite with heat pump, and I get alot of diagrams, but they're confusing as they show wiring it to an air handler. I'm just trying to figure out the wiring at the thermostat.
These wires are obvious-
Red goes to Rc, Green to the G, White to the W, and Yellow to the Y1.
So my questions are on what terminals these wires go to-
Blue, Black, and Orange