Gravel can generate in a bigger range than blackstone in the Nether

Blackstone can generate from layers 5 to 31 while gravel can generate from layers 5 to 41. This is because of the equation used.

Each has a "maximum", "top offset", and "bottom offset" value. Both have a maximum of 31 and bottom offset of 5. However, blackstone has a top offset of 10 and gravel has a top offset of 0. This leads to gravel generating much higher than blackstone, and pretty far above the lava ocean.