How to use mix blend mode in flutter?

Using RenderProxyBox and some painting, I was able to recreate the exact sample on the CSS website without asynchronously loading the image in your Flutter code.

Image using CSS (left) vs image using Flutter(right).

Read the article I wrote about this here

To start, a BlendMask SingleChildRenderObject is created that creates a RenderProxyBox render object called RenderBlendMask is created. The child is painted with a BlendMode and an opacity.

import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';

class BlendMask extends SingleChildRenderObjectWidget {
  final BlendMode blendMode;
  final double opacity;

  BlendMask({
    @required this.blendMode,
    this.opacity = 1.0,
    Key key,
    Widget child,
  }) : super(key: key, child: child);

  @override
  RenderObject createRenderObject(context) {
    return RenderBlendMask(blendMode, opacity);
  }

  @override
  void updateRenderObject(BuildContext context, RenderBlendMask renderObject) {
    renderObject.blendMode = blendMode;
    renderObject.opacity = opacity;
  }
}

class RenderBlendMask extends RenderProxyBox {
  BlendMode blendMode;
  double opacity;

  RenderBlendMask(this.blendMode, this.opacity);

  @override
  void paint(context, offset) {
    context.canvas.saveLayer(
        offset & size,
        Paint()
          ..blendMode = blendMode
          ..color = Color.fromARGB((opacity * 255).round(), 255, 255, 255));

    super.paint(context, offset);

    context.canvas.restore();
  }
}

Now to blend two widgets (Not restricted to images), just add the widget you want to blend above another using a stack, and wrap it in your BlendMode.

class ImageMixer extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SizedBox.expand(
          child: Stack(
        children: [
          SizedBox.expand(
            child: Image.asset(
              'images/sky.jpg',
            ),
          ),
          BlendMask(
            opacity: 1.0,
            blendMode: BlendMode.softLight,
            child: SizedBox.expand(
              child: Image.asset(
                'images/monkey.jpg',
              ),
            ),
          ),
        ],
      )),
    );
  }
}

That produces the image above, and it works exactly like the CSS example.

Try ShaderMask.
BlendModes

Example of use:

import 'dart:async';
import 'dart:typed_data';
import 'dart:ui' as ui;

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: SafeArea(
          child: MyHomePage(),
        ),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Future<ui.Image> loadUiImage(String imageAssetPath) async {
    final data = await rootBundle.load(imageAssetPath);
    final completer = Completer<ui.Image>();
    ui.decodeImageFromList(Uint8List.view(data.buffer), completer.complete);
    return completer.future;
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<ui.Image>(
        future: loadUiImage('assets/cara_img.png'),
        builder: (context, img) {
          return img.hasData
              ? ShaderMask(
                  blendMode: BlendMode.colorDodge,
                  shaderCallback: (bounds) => ImageShader(
                    img.data,
                    TileMode.clamp,
                    TileMode.clamp,
                    Matrix4.identity().storage,
                  ),
                  child: Image.asset('assets/code.png'),
                )
              : Container(
                  color: Colors.red,
                  height: 100,
                  width: 100,
                  child: Text('loading'),
                );
        });
  }
}

code.png:
cara_img.png:

Results: