Was the 1510 AM carrier signal on the air, but silent with no audio from the studio, or was the transmitter completely off the air (no signal at all)?
I've noticed more off-air periods on WMEX lately. Sometimes I find that their stream and app are still running the programming, but no audio (just a silent carrier) on 1510 AM.
I can't hear their 101.1 FM translator here in Somerville (unless I go out and park my car in a few certain south-facing hill spots), but at times when I've driven through its area, approximately from Dorchester down through the upper coastal South Shore, I've sometimes heard the FM translator still broadcasting while 1510 AM was a silent carrier, but sometimes I've heard both the AM and FM silent.
These periods seem to happen more often at times when the programming isn't live, I'm guessing maybe because there's no one on site at those times to correct something quickly. I've heard it go off at length late at night when the programming is from the MeTV Music Network (oldies) channel, and Saturday morning when you were listening (9-11am) is normally Jeff Lawrence's WMEX Top 25 Countdown show, which is pre-recorded.