Imagemagick Issue - transparency thats turning blue!
The reason it happens is that GIF only supports one transparent color whereas many images have a variety of transparent values ranging from 0 to 255. Since transparency is binary we choose values from 0 to 127 as transparent and values from 128 to 255 as opaque. A future version of ImageMagick will permit you to specify a specific color as transparent instead of it deciding based on the alpha channel.
See http://www.cit.gu.edu.au/~anthony/graphics/imagick6 for a discussion on how to make transparency work properly with GIF images and why GIF is a bad choice for transparent images and how PNG is the preferred choice.