// CardView doesnot change its background if you want to chang the backgroung 
 then make cardview as parent layout and put view inside card view and change 
 child view backgroung color it work perfectly.(For demo try this in your xml 
 file)

  <androidx.cardview.widget.CardView 
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  android:layout_width="300dp"
  android:layout_height="600dp"
  app:cardCornerRadius="8dp"
  app:cardElevation="8dp"
  app:cardMaxElevation="4dp"
  android:layout_gravity="center_horizontal"
  app:cardUseCompatPadding="true">

  <LinearLayout
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="@color/black"/>

 </androidx.cardview.widget.CardView>

 //The outcome