The blueness of a wound cleanseth away evil

The blueness of a wound cleanseth away evil: so do stripes the inward parts of the belly. Proverbs 20:30 KJV

Beatings and wounds cleanse away evil, and floggings cleanse the innermost being. ~ Proverbs 20:30 NET 

The KJV reflects the Latin source with a small addition. The NET is trying to meld the Latin, LXX, and Hebrew sources. 

93tc The verb מָרַק (maraq) means “to polish; to scour”; in the Hiphil it means “to cleanse away,” but it is only attested here, and that in the Kethib reading of תַּמְרִיק (tamriq). The Qere has תַּמְרוּק (tamruq, “are a means of cleansing”). The LXX has “blows and contusions fall on evil men, and stripes penetrate their inner beings”; the Latin has “the bruise of a wound cleanses away evil things.” C. H. Toy suggests emending the text to read “stripes cleanse the body, and blows the inward parts” or “cosmetics purify the body, and blows the soul” (Proverbs [ICC], 397). Cf. CEV “can knock all of the evil out of you.”

The Hebrew word, chabbuwrah, translated here and here only as "blueness" means "bruise" or "stripe".  The following Hebrew word, petsa,translated "wound" simply means "wound".  So literally the Hebrews says "bruise/stripe wound".  The Hebrew, Latin, or LXX makes no mention of the color of the bruise.  This was inserted by the KJV translators.

Apparently according to blinded studies, one can only date a bruise 50% of the time to a 24 hour period.["Dating of Bruises in Children: An Assessment of Physician Accuracy" Pediatrics] I was going to say that bruises turn blue on the fifth day.  However even bruises on the same person inflicted at the same time can age at different rates.  A human body is extremely variable.[Forensic Pathology 102]  I thought that this information was very interesting considering that I had received some training on dating bruises to recognize child abuse.

The proverb in question does not identify the target for the corporal punishment.  However the KJV version of this verse apparently was being used by an IFB pastor to justify bruising their children.  Blue bruises are apparently justified.  This pastor is sick.  If you ever bruise your child then you did it wrong.