Looks more like a standard "beauty mark" to me, but it's likely worth keeping the fish in a hospital tank under observation for a few days, at least. If you really want to be cautious, you could fresh-water dip (remember that blues are known for "playing dead" during a FW dip, so watch the gills for signs of movement) and follow that up with 7 days of Praziquantel (e.g.; PraziPro) treatment in a QT.