How do I blend PKCanvasView and a subview inside UIViewRepresentable?

Question

I have a regular PKCanvasView implemented inside a UIViewRepresentable in SwiftUI.

struct Canvas2View: UIViewRepresentable {
    @Binding var canvasView: MyPKCanvasView
    @Binding var toolPickerIsActive: Bool
    private let toolPicker = PKToolPicker()

    let onChange: () -> Void

    func makeUIView(context: Context) -> PKCanvasView {
        canvasView.backgroundColor = .clear
        canvasView.isOpaque = false
        canvasView.delegate = context.coordinator
        canvasView.minimumZoomScale = 1
        canvasView.maximumZoomScale = 4.0
        canvasView.showsVerticalScrollIndicator = false
        canvasView.showsHorizontalScrollIndicator = false
        showToolPicker()

        return canvasView
    }

    func updateUIView(_ uiView: PKCanvasView, context: Context) {
        toolPicker.setVisible(toolPickerIsActive, forFirstResponder: uiView)
    }

    func showToolPicker() {
        toolPicker.setVisible(true, forFirstResponder: canvasView)
        toolPicker.addObserver(canvasView)
        canvasView.becomeFirstResponder()
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(canvasView: $canvasView, toolPicker: toolPicker, onChange: onChange)
    }
}

Then I added an imageview when the layout happens:

class MyPKCanvasView: PKCanvasView {
    var imageView = UIImageView(image: UIImage(named: "cat"))

    override func layoutSubviews() {
        super.layoutSubviews()

        if imageView.superview == nil {
            imageView.frame = bounds
            imageView.contentMode = .scaleAspectFit
            insertSubview(imageView, at: 0)
        }
    }
}

Everything seems to work great. I have an image behind the sketch.

The last step now is I want to blend the two graphics with the multiple blend mode.

I have attemped:

override func layoutSubviews() {
        super.layoutSubviews()

        if imageView.superview == nil {
            imageView.frame = bounds
            imageView.contentMode = .scaleAspectFit
            self.layer.compositingFilter = "darkenBlendMode"  ///tried this
            insertSubview(imageView, at: 0)
        }
    }

was an example from another question How to get multiply blend mode on a plain UIView (not UIImage)

but it has no effect. How to do it?