The term blue collar has been around for several decades. Blue collar refers to a type of worker. The worker is typically one that performs manual labor for an hourly wage. Most workers are distinguished in the service sector. The other term for workers is white collar. These workers are the opposite of blue collar workers. White collar workers do not usually do any manual labor. These individuals work behind desks instead.
If you are a manual labor worker then you may be skilled or unskilled. If you are skilled it typically means you have had some sort of trade school. Engineers for example that are considered blue collar workers often have undergraduate degrees from universities. Typically these workers are in manufacturing, mining, building, construction, mechanical work, maintenance, or repairs. City workers who maintain the roads are considered blue collar workers.
The term blue collar comes from the early 19th century when a blue uniform was part of the dress code for these employees. The clothing was provided by the companies so that the workers could get dirty when working without ruining their own clothing. Coveralls were the most typical uniform for these workers.
As mentioned some workers can have a higher education, but a few of the blue collar workers entered their field with a high school diploma gaining experience through working with the company. Trade unions often regulate these workers to ensure they receive fair treatment from the company they work for. There are stereotypes of these workers in the US which are perpetuated by TV.