labelBackgroundColor: String

Sets the Crosshair Label background color. The value of labelBackgroundColor can be a “HTML Color Name” or “hex” code.

var  chart =  new  CanvasJS.Chart("container",
{
 .
 .
 axisX:{
     crosshair: {
          enabled: true,
	  labelBackgroundColor: "black",
	  .
	  .
     }
 },
 .
 . 
});
chart.render();