Blood is always red. The walls of veins appear blue, but it is a very bright - almost cherry red color after being oxygenated by the lungs (why do we have blood?) And then goes to a dark maroon after it has been depleted of oxygen and is carrying CO2.
If you want to see blue blood, then you have to go to some species of shellfish and other sea creatures which use a copper-based molecule where we use hemoglobin (which is iron based). It is inefficient at atmospheric pressures, but as you go into the deep depths of the ocean, the higher pressure makes oxygen toxic through our normal respiratory cycle. Copper based blood is one way ocean creatures avoid this.
But I digress.
Purple was a difficult color to die clothing in the centuries prior. Purple dies often came from abroad and were expensive. Purple was almost exclusively worn by the aristocracy and ruling castes in Europe. The power of the industrial revolution and free market-ish strategies reduced clothing color to an arbitrary choice for the masses. Now we can wear kings' clothing while being mere scullery servants in the fast food industry.
Consider how different these people see the world. To them, we are generally like animals to be kept. To see us parading about with their symbolism is seen as somewhat quaint, or a crime in and of itself - a breach of what they see as the rightful order of society.