You need the schematic to know if b or g is possibly the ground connection, also known as the 'common' designated usually by the letter C. If you can't find the schematic and have an available screw on the metal furnace frame, you can use that as ground. You may see a green wire attached to the furnace frame... again, that's the ground.
On the S86 control you have the 24 V and 24 V (GND) connections. The 24 GND (ground) will attach to the furnace frame... the ground. But the other 24 volt terminal on the S86 may be empty, as it was in my situation. Check page 5 carefully.