I'm starting to suspect the smelting ratios don't apply to boulders at all. I have smelted three batches of 25 Black Ore and one batch of 20 Black Ore, all mined from boulders;
according to the wiki it should have an ore-to-bar ratio of 30%, but two of the 25 batches and the 20 batch all netted one bar, while another 25 batch netted only Gneiss. Meanwhile, I have been smelting Cassiterite (also listed as having a smelt chance of 30%) mined from a cave node, and I'm consistently getting at least 5 bars per batch.
It's really frustrating, as you can barely do anything with metal until you've made an anvil + hammer, which costs 7 iron bars. I'd need to find a third, and probably even a fourth boulder to get that.