Dilate Image over Alpha?

[Raildex]

How do I dilate an image when I want to use the Alpha as a "mask"?
I want to achieve Padding, similar to this:

I have tried

newImage.Morphology(MorphologyMethod.Dilate, Kernel.Disk, Channels.Alpha, 4);
newImage.Morphology(MorphologyMethod.Dilate, Kernel.Disk, Channels.RGB, 4);

In the case of Alpha, it results in a black border around my image.
In the case of RGB, the image itself becomes very blurry.

Here is the image I want to dilate

and this is the result after
newImage.Morphology(MorphologyMethod.Dilate, Kernel.Disk, Channels.Alpha);

And here after I use Channels.RGB:

[dlemstra]

I will wait for the answer you are getting in the ImageMagick discussions. And then I can help you translate that to C# if you need help with that.