Create transparent image with background in node.js and graphicsmagick

Question

How can I create the following effect using GraphicsMagick in node.js (using the gm library)?

• read image from buffer
• set a background color
• blur
• opacity
• return new image as buffer

What I am trying does not work:

gm(buffer)
    .background("#ff00aa")
    .blur(10,5)
    .operator('Opacity', 'Assign','30%')
    .resize(width)
    .toBuffer("JPG", function (err, buffer) {
        ...
    });

What I want is this result:

Answer 1

Managed to solve it:

gm(buffer)
    .out("-matte")
    .out("-operator", "Opacity", "Assign", "90%")
    .out("-flatten")
    .out("-background", "#ff00aa")
    .blur(100,30)
    .resize(width)
    .toBuffer("JPG", function (err, buffer) {
        handleResponse(deferred, err, buffer)
    });