the sudden short circut to earth when the mechanic hit the starter terminal could have simply fried or damaged a cell in your battery,this would be enough to cause you problems when a load is demanded of the battery.the blue wire is the charge indicator wire to the red dash bulb.i can assume its going out when the engine starts,a quick check with a hydrometer will indicate if a cell is down,if you cant access the cells then i would sugest a quick change of battery to see if this fixes you problem,
if not then look at the alternator charging end,as big short circuts can damage the diodes within the alternator giving rise to battery drain and similar symptoms