This is why the GP linked a source with the national average for college graduates and non-grads. Certainly there are tradespeople who make a ton of money, but usually they make less than degree holders.
There are many different trades, with different income expectations. And of course if you are willing to own the business (not easy) is a factor, some business are more conductive to owning your own business.
We need to be honest with kids: it matters what degree or job you presue. While I can't predict the future perfectly I can look at trends and say some engineering jobs are better than others. Med school looks really good too. Music on the other hand should be a second major or a minor if you study it at all. Likewise in the trades some are better than others, though I'm not sure what to get into.
I wonder how much of this isn't also driven by the reduction in private union membership. I've worked white collar jobs in organizations with strong unions and I'm willing to bet the blue collar workers were probably almost surely, on average, more than the average white collar workers elsewhere. And when I worked in areas with weak union membership, the converse was true.
The difficulty in the former was that it was hard to get into the union, but once you did, you were probably making many multiples of the average household income for the locale.