I found a bug of Labels in Cesium 1.30. When I swithed to 2D,the label shows another black label.
I added a property of background and set the alpha 0.0 in Label.html , that could help me solve it.
function addLabel() {
Sandcastle.declare(addLabel);
viewer.entities.add({
position : Cesium.Cartesian3.fromDegrees(-75.1641667, 39.9522222),
label : {
text : 'Philadelphia',
Hmm okay. What video card does your computer have? It could be related to that.
Have you tried running in incognito mode with chrome? Sometimes things get cached when switching between versions of Cesium and that can cause unexpected things to happen.