I just saw that House Bill 1772 was just passed yesterday, You now can paint a Purple Stripe on trees/post to show you property is posted instead of putting up No Trespass signs.
The reason for the Purple paint is the signs degrade over time or get torn down.
To take effect in 60 days.
I believe blue is used in the logging industry to mark trees for harvesting for lumber and firewood. so not to confuse the two between line marking and for cutting trees . I could be wrong. just a guess