In Blogger, you change your view from "Compose" to "HTML". Then before the text you want stricken with a line through it, type "<" then either "s" or the word "strike", then ">" and at the end of the text you want with a line through it, type the same thing, only put a "/" before the "s" or "strike" in the closing set.
So it would look like this, only with no spaces in the string of characters:
< s >mistake< /s >
Cool!