So, I answered my own question but will leave it up in case anyone else wants to know. Instead of using hex for the color of the background, I used HSLA format.
#block-d325a806d44c4257f1d4 {
background: hsla(0,0,0,.15);
padding: 1px;
text-align: center;
}
For anyone that doesn't know, the last number (.15 in this case) represents the opacity of the background color. This way it will only affect the background color and not the whole block, thus the text stays opaque while you can freely change the transparency of the background.
