Wow. Ammonia isn't the only toxic thing in this thread.
Pay them no mind, OP. I am on the "your ammonia spiked after the blenny died" team. I don't think you have any kind of stuck cycle based on the evidence presented. Not sure if you're running any chemical filtration but in a small tank like this, chemipure blue is a really good insurance policy. Maybe the blenny needed more food, or maybe it was something else. You mentioned it was a pretty recent addition. Could have easily come in with something. Because your tank is pretty small, I suggest that you buy one of the sealed 5 gallon containers of premade salt water that they sell at the big box pet stores and keep it as a backup. In the future WHEN disasters strike, it will be very nice to be able to easily change 3 gal or something.
As the others said, the API tests are not the best, but honestly, they should be close enough for a simple set up as long as you're not going for high end acros or something. People love to push the better test kits, and don't get me wrong, they are considerably better-- but people can't even agree to what the levels should be these days. If your nitrates are 5 or zero, it's probably a good plan to feed your fish more. If they are 20 or 30, it's probably a good idea to get to that water change sooner than later. If people knew and agreed that "x" was the perfect nutrient level, then sure, test down to as many significant digits as you can. But this sounds like a low tech set up, and a ball park is probably close enough for many entry level corals.