I think I left them in a place with not enough airflow and they started growing black mold.

Will cutting the segment off make it safe to consume?

If so, how much should I cut off?

If it isnt systemic and all throughout the cact but just a superficial infection, yeah cut out all the moldy part and a bit more just to be safe and youre likely fine.

Show pics of how it looks after cutting it out.

Thanks for taking a look, I cut off 1/4 inch on the big one and its been drying for almost a day and looks pretty healthy.

Don't think this is systemic, since this is a middle cut, only the bottom cut side grew mold, not the top. I think this is because I foolishly stored the cactus upright for a week, so the bottom was getting zero airflow.