Hello and welcome to HCM,
from your history and description it seems to be a bruising of the skin possibly due to pressure applied on the back, during some strenuous activity, or heavy weight on the back. SInce it is completely without pain or itching, and appeared suddenly, this seems like the first possibility.
The skin becomes delicate and easily bruises with age.
Do consider whether you have any other symptoms like
fever,rashes elsewhere, bleeding, or if you are on any blood thinning meds etc.
Bruising will slowly turn darker and fade . you can apply thrombophob ointment to help.
But if you notice it to be spreading, or any new symptoms do
consult a dermatologist to rule out infections or other lesions, and the need for testing the area.
Hope this helps