Hi, I'm not in the business of telling anyone who they are but your Grandmother could have Native American ancestry because some Native Americans were black. Like the Choctaw. Here's a link: https://en.wikipedia.org/wiki/Choctaw_mythology#/media/File:Choctaw_group.png

I only share this because I know this so well being from the area (Louisiana). It's a part of my grandfather's ancestry. It wouldn't register on a DNA test because who we traditionally know as Native American have a Ancient North Eurasian populations and an ancient East Asian population. While the Choctaw have a Southeast Asian and African lineage.