Michael Blackson thinks Elton John had two choices while shoe shopping in France when nature called ... pee in a bottle or explode!!!

We got the comedian at LAX and our photog asked him about the French storeowner claiming Elton came into his shop and pissed in a plastic bottle in the middle of the store.

Michael says Elton must have a weak bladder at 77 years old, and this could have actually been a medical emergency ... with dire consequences if EJ didn't release his flow.

As we reported ... Elton was allegedly shopping for shoes when he asked the store owner for a public restroom and then peed in a bottle when told there wasn't a toilet around, allegedly getting a few drops on the floor.

Michael says the store is lucky Elton got any fluid in the bottle and didn't soak other shoppers and the Sugarkikz merchandise.

It's a funny conversation ... and Michael tells us why he's no stranger to turning a container into his own personal toilet.