Make the 8-spice blend: in a medium bowl, combine coriander, garlic powder, onion powder, cumin, paprika, chili powder, oregano, and cayenne pepper. Whisk to combine and set aside.
Prepare the tempeh. Cover the bottom of a medium saucepan with about 2 inches of water. Set the saucepan, covered, over medium heat and bring to a simmer. Once simmering, arrange the tempeh blocks in a steamer basket. Set the steamer basket in the saucepan and lose the lid. Steam the tempeh for 10 minutes.
While tempeh is steaming, make the marinade. In an upright blender combine 2 tablespoons of the 8-spice blend, balsamic vinegar, water, oil, lemon juice, tamari, agave nectar, salt, and pepper. Blend on high until completely emulsified, about 15 seconds.
Arrange the steamed tempeh in a shallow dish. Pour the marinade over the tempeh, making sure that as much tempeh as possible is exposed to the marinade. Cover the tempeh and marinate for at least 1 hour (or up to 8 hours in the fridge), turning and flipping over the pieces at the halfway point.
Once tempeh is sufficiently marinated, preheat your grill to medium. Clean the grates. Set the pieces of tempeh on the grill. Cook the tempeh until char marks begin to appear on the underside, about 5 minutes. Carefully flip the tempeh pieces over and cook until the other side has char marks as well, another 5 minutes. You can brush remaining marinade over the tempeh as it grills if you like. Serve tempeh hot with accompaniments.