The FloatingActionButton class from AndroidX uses the colorOnSecondary theme attribute to tint its icon.
https://github.com/material-components/material-components-android/blob/master/lib/java/com/google/android/material/floatingactionbutton/res/values/styles.xml#L39
If you follow the MaterialComponents theme definitions down into the base definitions, you'll see that the default value for colorOnSecondary is design_default_color_on_secondary... and that is defined as #000000.
https://github.com/material-components/material-components-android/blob/master/lib/java/com/google/android/material/color/res/values/colors.xml#L26
You can fix this by either adding the app:tint attribute directly to your FloatingActionButton or by re-defining @color/colorOnSecondary in your theme to be whatever you want.
