"Inca blots" sounds like "inkblots". The Rorschach test is a psychological test in which subjects' perceptions of inkblots are recorded and then analyzed. The Inca civilization existed in ancient Peru beginning around 1400 CE. This joke is playing on the word "Inca" because it sounds like "ink" and it references people that once lived in Peru.
What do you call Peruvian Rorschach tests ? Inca Blots .