I recall reading that if an AM radio is tuned to frequency x plus or minus the heterodyne frequency a radio tuned to frequency x will be blotted out. Is this true? If so what is the heterodyne frequency? Will it work with FM?
The local oscillator (L.O.) in an AM band set is 455KHz away from the carrier - usually higher.
The IF for FM is (or was, lo these many years ago) 10.7 MHz, and the L.O. is usually higher.
Yes, if you have a really crappily-designed AM receiver, tuning it to a freq. 455KHz below the carrier of the station in question can interfere with its reception. But they _should_ be designed to not emit any L.O. signal, for this very reason.
On FM, not so much, if at all, because of the different characteristics of the modulation/demodulation technology.