I agree with everything you said.. Not backed by anything yes.. But so is everything else, it's just the trust we put in it.

(I am not an expert but I am doing my best in what follows)

How can blockchain be easily described to the average person?

A relatively new way of having a database replicated everywhere in a secure way using building blocks (employing state of the art encryption) as a way to establish peer to peer trust without a centralized authority.. The peers are going to agree somehow (either vote or use powerful nodes) on if a change to that database is legit or not.

Does the blockchain make it safe?

Now it seems to be the safest, we will wait for the next Satoshi to bring a better alternative.. But now this is what we got.

What is the backing of cryptocurrency?

Nothing. But so is every other currency.. And so is the gold.. I mean paper currency is a piece of paper that you trust others will take from you in exchange for goods.. Some people argue that gold is different (in some way it is as it's more portable and cannot be printed) but you just trust others will take from you in exchange for goods, if the free market decides it's useless then it will die off.

but it can still fall apart as it is not married to enough solid assets.

Having governments using and stoking BTC is enough insurance to go full on it for me.

Have you heard the stories of people transferring tens of millions in usd using LTC and yet paying <$1 for the whole transfer that takes a second? how much fees does it even take to buy a $100 item? what about transfers and limitations, add to that censorship, the government will rob you if you have the wrong thoughts, now imagine if this currency is not controlled by the government and if it cannot be inflated for whatever reason.. Just think about it, we have been scammed by the current financial system for centuries.. Governments are bad at handling currencies..