Congratulations on landing the jump!
Is the broken screw steel? (If so, it would stick to a magnet. Aluminum screws don't rust, but may not take as much stress. Oddly enough, the highest grade marine stainless steel screws aren't very strong either, because there is a trade off between strength and extreme rust-resistance.)
Is the broken screw significantly rusted?
If it is steel, and not very rusted, you must be incredibly strong!
I guess in the long run, Bill_S's suggestion of using a good grade stainless screw, in a larger diameter, makes sense, even if you use the skate tech for a fast repair. Especially if the same skate tech mounted the blades with a poor quality screw in the first place.
Of course your boot soles and heel should be well waterproofed - that might help a little. What brand of boot was it?
It would be interesting to know what type of glue the skate tech uses to repair the heel. I've played a little with "Shoe Goo", but that might be a U.S. brand, and I'm not certain if it is as strong as the original leather and the glues that commonly hold heels together.