The blue color comes when the metal just starts to glow red. You have to remove the flame quickly at that point or else it'll start to turn that greyish color. It's a very sensitive process, and you can't leave the flame in one spot then move to another and think it'll all heat and color evenly. When I do it, I stare very closely and examine as the color is changing and apply or remove heat as necessary to every spot on the metal to try and blend it all and keep it as even as possible.
So basically, as you're torching/watching one spot, remove the flame, check, see what the color is, and apply flame again if necessary to achieve desired color. One thing to note, the color appears faded when the flame's directly applied. When you remove the flame, you'll notice the color gets more vivid. So it's important to keep checking the areas you're torching by removing the flame.
Ideally, a kiln is by far the best method. If I had $300 laying around, I'd get myself a digital table top kiln. Interior dimensions are 4" x 2".