This term has a sterotypical connotation based on historical perspective.
The origin of the words refers to the dress codes found at their respective workplaces. Industrial blue-collar workers formerly and to a large extent still do wear "work clothes" where the shirts are a navy blue color. The clothes are more durable and intended to get scraped and/or dirty while working. Part of the dress code may feature protection from work-related injury. Examples are hard hats, heavy work boots or steel-toe boots to prevent injury to the feet, in contrast to white-collar workers where wearers of the traditional white, button-down shirt were not intended to do physical work or soil their shirts.
Blue-collar is also an epithet derived from the "blue-collar worker," used to describe the environment of the "blue-collar worker": i.e., a "blue-collar" neighborhood, job, factory, restaurant, bar, etc., or a situation descriptive of use of manual effort and the strength required to do such. It can also be used derogatively as an adjective to describe something crude, simple, lacking sophistication, or appealing to basic instinct: i.e., a blue-collar joke.
Some distinctive elements of blue-collar work are the lesser requirements for formal academic education. Training is often learned on the job while working. The boss of such workers is usually called a foreman whose duty is to assign and monitor the work of his subordinates. Commonly the foreman is a manual worker himself or a "working foreman" who in turn is subordinate to a higher boss. Another aspect of blue-collar work is the time clock used to record the precise hours that the employees work and therefore calculate their pay- which is usually based on an hourly rate and paid weekly. Generally, the hours of such occupation are strict (see shift work). But after "punching out" (a process of recording the time leaving the company at the end of the day), it is understood that the worker has no duties until the next day.
Generally, the pay for such ocupation is lower than that of the white-collar counterpart, although higher than many entry-level service occupations. Sometimes the work conditions can be strenuous or hazardous.
Commonly the "blue-collar" worker will be part of a labor union which is a form of organized labor. These associations use a process of negotiation called collective bargaining to establish the rights and responsibilities of the workers, to negotiate the pay rate and benefits received. Also, there are laws and organizations that regulate safety in the workplace, associated with "blue-collar" conditions.