Okay, I did some fusible applique. I'm getting ready to stitch around the edges. I'm planning on doing a blanket stitch rather than a zig zag. I know to get a complimentary top color (I have probably 30 different colors that have been fused on, so I know I'll need lots of thread colors). Does the bottom thread matter? Is it like regular quilting where you want to match in case in peaks through? Or is the blanket stitch somehow different where I can use a neutral thread and it will be fine? I don't want to start winding lots of bobbins for a quick little bit of stitching. Thanks!

I just use white in my bobbin

You'll have to see and play with your tension. If your bobbin thread shows through and you see it... Usually, I just adjust the tension to hide it and can use a neutral.

I usually just use a neutral as well.

Neutral here too.

phew! I feel relieved!

I usually use a light gray...it blends in almost invisible with the lighter colors.

I either use the same color as the background fabric or the same as the applique. It really depends if the bottom stitch is shown on the top or not. I am doing the same as you machine applique and since most of it is in light colors I am using white or off white for the bobbin thread. Try it out on scrap first to see if the bottom thread is seen or not.