I think you're being precious OP, as only people with children can be about their offspring (not that that's shared by all parents at all, mind you, as evidenced here on this thread by some posters with actual self awareness).
I can forgive a toddler not reading social situations, adults are the real problem I find.
Which I suspect is exactly what happened here. You missed some important social cues and body language signs from mum, before she blocked your child from following alongside them. Eg. avoiding eye contact, grimaced or 'forced' smile' with lips pursed, eyebrow flash whilst force smiling, turning body slightly away, speeding up slighty, even just obvious and blatent ignoring, is a very important sign. You missed one.
I remember once shopping in asda. Was looking at the sweetie aisle, was very close to the shelf of the items I was choosing from. A child climbed in between me and the shelf (there was barely any space, I was quite aghast at his efforts to get between us). He wasn't even quickly grabbing a sweetie. I looked behind me to the mother who was just stood there like some sort of empty husk, I gave her a look but genuinely she just was blank in the eyes and didn't even seem to realise the rude behaviour of her son. There wasn't even a sense of entitlement in her expression, just pure stupidy looking back.
Sometimes parents aren't with it (and I don't blame them). I suspect this was you in this case, that or the entitlement that people should want to interact with your son no matter what they're doing.
She doesn't owe you an explanation of what she was doing, if she was busy and in a hurry, if her child doesn't play well with others, if he's ill - nothing. It's none of your business - she's a stranger out shopping.