Ok back after further tests, some observations :
Problem appear with retractions, but blob are not where retractation take place, they are rather where retractation lands.
Indeed my goal is not to print a simple cube, I could achieve a nearly perfect one by simply disable retractation, but rather print a piece with retractation without any blob, so that I will be able to print several parts together with nice finish, or print multi-materials without blobs artifacts.
For now the best solution I have found is to set 3 perimeters (with only 2 perimeters it still renders thoses little stairs), with inside-outside direction. This way blobs are far less visibles but still not perfect...
I will continu to investigate and will give some more feedbacks. If someone find another or better solution pleeease share it.