Chartjs-ChartJS custom tooltip doesn't render background on labels (only the title)

There seems to be an issue with the custom property.

I recommend using the callbacks instead :

tooltips: {
    displayColors: false,
    backgroundColor: 'rgb(0,0,0,1)',
    callbacks: {
      title: function(tooltipItems, data) {
        return 'Hello';
      },
    }
}

See jsFiddle