com claims that H-2A and H-2B are used to import blue collar and agricultural workers. Is it true?
Yes. Both programs are rarely used because of the excessive paperwork, but both are indeed intended for this purpose. The US government actually encourages use of the H-2B program to curb illegal immigration. Update: recently, the H-2B program has gained in popularity, and today, the maximum number of H-2B visas per year under the quota is being issued. This is still only 66,000 workers per year.