From Breiss' website:
Crystal 10L - "A pale caramel malt that contributes golden color and mild caramel flavor."
Crystal 40L - "A medium domestic caramel malt that contributes golden to light-red color and caramel/toffee flavor."
Crystal 50L... by "50L" did you perchance mean "60L?" Here's for 60L: "A medium domestic caramel malt that contributes golden to light-red color and pronounced caramel flavor."
So really, the difference is going to be a slightly more pronounced caramel flavor, which is up to you to decide if that's what you want in your blonde ale. How much Crystal 10L does it call for? And is it an all-grain or extract batch?