During the research I did, Ballota nigra seemed to always be referring to black horehound, but both horehound and white horehound were always Marrubium vulgare. Black horehound looks different. It's leaves don't have that silver look to them, and I don't think they're fuzzy like white horehound.

I'm just this year getting into companion planting. I've been interested and reading about it for a few years. But I'm just coming to a place where I'm comfortable trying it out. I dont think I've come across the tomato/horehound relationship. Thanks for the tip!