Step 1: import dart:ui as ui

 import 'dart:ui' as ui;

Step 2: Wrap the container behind where you want to add blur effect, adjust sigma for increasing or decreasing blur effect.

ClipRRect(
        borderRadius: BorderRadius.circular(25),
        child: BackdropFilter(
          filter: ui.ImageFilter.blur(
            sigmaX: 4,
            sigmaY: 4,
          ),
          child: Container(
            width: 40.uw,
            height: 206.5.uh,
            decoration: BoxDecoration(
              borderRadius: BorderRadius.circular(25),
            ),
          ),
          )),

Output: I have kept color transparent the example output