Yo, thank you to yogurt, sourdough, and various fermented beverages. Bless.

It sounds like a few different things got mashed together there. Ergot is a hallucinogenic fungus that grows on rye, and is speculated to be the cause of some of the witch panics. It's not the same fungus found in Roquefort, but it is what they use to make LSD.

Witches flying is hypothesized to be entheogen use, since a common side effect is feelings of floating, flying, or otherwise 'being high'.

The fungus you're thinking of is likely ergot, because it shows up in pretty large volume in batches of rye.

In processing, it ends up as a dark purple/black dried up mass that assumes kinda a crescent shape. Mills will run a batch of rye through a color-sorter - a bunch of times consecutively - to reduce the amount of ergot in the batch before milling.

You can technically refine it into LSD, but if you screw up, you can kill people. (Morning Glories are the preferred method).

The number of 55-gallon drums of ergot I've disposed of, though... It's difficult not to identify with Walter White and wonder... "what if?"