This isn't really a Blockly question, but a basic CSS question. Unfortunately, this week it is my turn to answer your questions, and I happen to be the one member of the Blockly team who knows the least about CSS. Fortunately, however, I have MDN to show me what to do, and my helpful teammates to explain why the thing I tired at first wasn't working!
After a bit of experimentation with the Chrome inspector, I think you can achieve the desired result by adding a few extra items to your stylesheet:
.blocklyFlyoutLabel.RGBBulbStyle>.blocklyFlyoutLabelBackground{
opacity: 1;
fill: #00f;
}
(You don't need to put a fill on the blocklyFlyoutLabelText.)
You can also set rx and ry to 0 if you don't want rounded corners.
Apologies again and best wishes,
Christopher