The image above shows one decal fired on White Glass under clear (the pumpkin) and another fired on French Vanilla under clear (the witch). I'd bet the new Reactive White (0009 Reactive Cloud) would also make the decal look sorta black.
At the Aquila Glass School you can get Don to print out your images onto "Photo Fusing Paper" (he has the paper and a laser printer) and many people use this product to put images into their fused glass projects. They bring in their image saved onto a memory stick and he prints it out onto the decal paper. You wet the printed paper and slide the decal onto your glass. The results can vary, but its a simple and effective process.
The iron oxides in the laser toner make the image withstand high temperatures of firing the glass, afterwards it leaves a sepia-tone type image (sort of a burnt rusty color).
Here is a link for more information and prices for having Don print your decals:
to check out Laura work: