That's because blue-collar jobs, especially in the trades, pay what has been known as "family-wage" pay. That was a notion that became popular in the earlier part of the twentieth century, with the rise of labor unions. The idea was men were the primary breadwinners of families, while their wives stayed home with the kids, and therefore men should be making enough to support their families. Of course, it wasn't just limited to blue-collar jobs; the whole sexist idea that only men deserved higher pay was the rule throughout the economy. The notion of a "family wage" sounded good, unless you were a woman who needed to work, especially if you were a single woman. Women not only made less money in the same kinds of jobs as men, but jobs that women dominated, such as K-12 teaching, nursing, and clerical work, paid "secondary" wages or salaries. Women just worked for "pin money," after all, not because they HAD to.
"Experts" over the years tried to make phony-baloney arguments as to why women did and continue to make much less money than men, such as their education levels, workforce participation, and the like. However, the "family wage" concept is the REAL reason traditional women's work has been denigrated, even by so-called feminists who should know better. Women are assumed to be supported by men, and God forbid if you remain single and opt for a career in a field like education. You might do better than women who are in jobs not requiring a degree, but you still can't buy a house and fund a decent retirement on a teacher's salary. You still have to have a man support you financially to have a semblance of a decent standard of living.
Given this reality, this article falls short of the argument because teaching is NOT a blue-collar job. Blue-collar jobs by definition do not require a college degree and they are not considered part of the professions.
The author needs to read Louise Kapp Howe's 1977 classic on "women's work" called Pink-Collar Workers. Howe sadly died in 1984 of cancer, aged only 49, so she wasn't able to do a follow-up or a sequel to her classic work. Had she lived, she likely would have written about the decline of the standard of living for BOTH men and women. The "family wage" concept is almost totally extinct because businesses found that by flooding the labor market with women workers, they could in "good conscience" pay men far less so that both adults in a married household would work for what one worker used to be paid. Businesses also successfully undercut labor unions, so workers had even less say over their pay, benefits, and working conditions.
I fully expect child labor laws to be repealed if public education becomes completely privatized. Those parents who can't afford a private school education for their first-graders can send those kids off to the sweatshops of tomorrow.