You can also mix approaches: optional semicolons, but use indentation to guess if it's the same instruction or not. That way:

// 3 instructions
blah; blah
blah
// 2 instructions
blah blah
blah
// 1 instruction (indentation is significant!)
blah blah
blah
// This one is tricky. Id' say syntax error, or a warning
blah; blah
blah
// This looks better, more obvious to me: 2 instructions
blah; blah
blah
// Alternatively, this one may also count for 2 instructions
blah; blah
blah
// begin of a new block
appropriate_keyword blah blah
blah
// end of a block (one instruction in the inner block,
one instruction in the outer block).
blah
blah
// 2 instructions (but frankly, I'd provide a warning for the superfluous ";")
blah;
blah;

This should be flexible enough and unambiguous enough.