Why can't I declare functions or variables at the block statement?

Question

In the function, running normally, this displays "Hello world":

function fn () {
    function b1 () {}
    var b1 = 'Hello world';
    console.log(b1);
}
fn();

At the block statement, get an error: Uncaught SyntaxError: Identifier 'b1' has already been declared

function fn () {
     if (true) {
         function b1 () {};
         var b1 = 'Hello world';
         console.log(b1);
    }
}
fn();

So there is clearly something "special" about a block statement with respect to function declarations. why?