I belive it is based off a "magic trick" called Black Magic. To proove that they are communicating telepathicly, they follow a rule:

>! The item listed after the black item is the item listed.

(Hence the name, Black Magic).

Each item before the last one fits the criteria for Black Magic to work. 

In the first one: 

> Jasper points at a lime. When Alex returns, Boris asks, “Is it that napkin?” Alex says, “No.” Boris asks, “Is it the night sky?” Alex says, “No.” Boris asks, “Is it that lime?” Alex says, “Yes.”

The night sky is usually a black object (without stars and stuff), so it fits the criteria for Black Magic.

In the second one: 

> Alex leaves the room again, and Jasper points at an ashtray. When Alex returns, Boris asks, “Is it that spoon?” Alex says, “No.” Boris asks, “Is it that cigarette?” Alex says, “No.” Boris asks, “Is it your shadow?” Alex says, “No.” Boris asks, “Is it that ashtray?” Alex says “Yes.”

A persons shadow is colored black, so it also fits the criteria. 

In the third one:

>This is repeated again and Jasper points at a straw. When Alex returns, Boris asks, “Is it that cigar?” Alex says, “No.” Boris asks, “Is it that fork?” Alex replies, “No.” Boris asks, “Is it that rose?” Alex says, “No.” Boris asks, “Is it that crow?” Alex says, “No.” Boris asks, “Is it that straw?” Alex says, “Yes.”

Crows are known for being black birds, so it too falls under the same criteria. 

Alex just has to make sure he notices the item that is black that Boris states, and he will be able to "telepathicly" figure out which item is the correct item. 

There are other ways to do it, like the nth "is it __ ," but the Black Magic way allows it to be much more flexible, and could either be fast or short. Just not on the first one, while the nth "is it __ " way could apply to the first one (if said so before hand), but that could cause it to be dull.