Hm, that is weird. Maybe you didn't have enough of the color on the block to begin with. The way you described is exactly how I learned.
The fabric crayons work a little differently: You draw the design on paper and iron it onto the fabric. I tried that once and it came out really light as well.
Can you re-color? I would get a set of fabric markers (NOT Sharpie!) and color the designs. I have some that work like felt tips.