Why is i2cset giving "resource busy" on my BeagleBone Black?
It ended up being the second i2c bus that I needed to use.  As you can see below, 0x70 is the "chip address" available on bus 2!  UU is signifying something unavailable or busy.  



After this I was able to follow the nice instructions here, but using bus 2.  

Specifically,

i2cset -y 2 0×70 0×21

i2cset -y 2 0×70 0×81

i2cset -y 2 0×70 0xe0


for starting the multiplexing oscillator, enabling the display with no blink, and brightness

Using this information I was actually able to modify Adafruit's code for matrix8x8_test.py example, explicitly defining the i2c bus and chip address display = Matrix8x8.Matrix8x8(address=0x70, busnum=2).  This is on a BeagleBone Black wired exactly as Adafruit said.  I hope this helps someone.  I will definitely be learning the i2c bus inferface myself as it's going to be worth it in the future when stuff like this comes up.