Generally blocking traffic to protest is illegal. Whether it is moral or not is a different matter. From the FindLaw blog:

Blocking traffic is an illegal, albeit effective method of protesting in the United States. When protesters block traffic, they are engaging in civil disobedience, a term coined by one of America's earliest freethinkers and intellectuals, Henry David Thoreau.

Here is the Washington ACLU saying something similar:

Can protesters march in the street?

Yes, but… many cities require permits if a march will block traffic; the rules for obtaining permits and enforcement of permit requirements vary by jurisdiction. If you march without a permit and block traffic while refusing an order from police to get out of the street, you could be arrested or detained. If you intentionally block traffic as an act of civil disobedience, you could face criminal charges.

Do I need to get a permit for my protest?

It depends. The government is not allowed to require permits for protesting, leafletting, or signature-gathering on public sidewalks when these activities don’t hinder traffic, create a risk to public safety, or compete with others’ uses beyond the ordinary use of the space. [1] You should check your local ordinances for specific requirements if you are planning a large event that will block vehicle or pedestrian traffic. If a permit is required, the rules for obtaining a permit must be laid out in advance and cannot be different based on the content of the speech or the focus of the protest.