Flutter Card with Transparent Background

try setting the elevation to 0, it should work

Card(
      elevation: 0,
      color: Colors.transparent,
      child: Padding(
        padding: const EdgeInsets.all(16),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            CardLabelSmall("Current Premix Plan Document"),
            Expanded(child: PremixPlanDocList()),
            Row(
              children: <Widget>[
                Expanded(
                  child: RaisedButton(
                    onPressed: () async {
                      homeBloc.retrieveCurrentMrfPremixPlan();
                    },
                    child: const Text("Retrieve Premix Plan"),
                  ),
                ),
              ],
            ),
          ],
        ),
      ),
    );

You can try something like this.

  new Container(
                height: 300.0,
                color: Colors.blue,
                child: new Card(
                    color: Colors.transparent,
                    child: new Center(
                      child: new Text("Hi modal sheet"),
                    )),
              ),