One relatively straightforward "rule of thumb": Look for the semi-colon(s). Referring to your example, starting at the "while", read forward until you see a semi-colon. If you want anything beyond that semi-colon to be executed as part of your while block, you need to wrap it all in curly braces.
Another way to look at it: The semi-colon is a statement terminator. It terminated blank = 0 as your previous statement; it terminates not only the if but also the enclosing while statement. Thus, to execute the following if as part of the while block, you need to enclose the ifs in curly braces.
Oh, and by the way, C and similar languages do not attach syntactic meaning to whitespace. It is at most treated as a separator. Any indentation you choose to apply is for the benefit of the human reader only; it has no significance to the compiler.