You need to be bridging CS to BCM17 on the BlackHAT and thus the Propeller HAT to CE0 on the Explorer HAT. This is looking good though, I might have to try it myself!
Done! And it works, too. At least I have it detecting the Propeller, I haven't tried uploading anything yet though.
Note- the TX/RX labels on Propeller HAT are reversed so you need to connect TX from the Black HAT to RX on the Explorer HAT Pro, and RX on Black HAT to TX on the Explorer HAT Pro. You should have a total of 5 wires running to the Black HAT- 5v, GND, Reset, RX, TX. These are physical pins 2, 6, 8, 10, and 11.
You can connect a Trinket up at the same time, too, if you use the PWM ( GPIO 18 ) as reset for the Propeller, and CS (GPIO 8) is the reset for Avrdude.