Custom blocks cast shadows onto themselves

Blocks (more noticeable on custom block models) cast shadows onto themselves, or they just have dark spots on them, noticeably near the bottom of them. In this example, I used a Pig model and a custom plant-like model (with an edited gunpowder texture on it). The Pig's back legs' fronts are dark, and the whole gunpowder plant-looking thing is dark (I'm holding the item as a comparison). The other example includes a chicken model (with dark feet).

Steps to Reproduce:

Make a custom block model and put it onto a custom block.

What I expected to happen was...:

The model should be nice and bright.

What actually happened was...:

The model has a dark point(s) on some portions of it.