[sqlite] Why is empty string not equal to empty blob?

[Paul]

Maybe this question was already asked and explained.
Or maybe it is documented somewhere (could not fiund it).
Sorry, if this is the case, but why does

SELECT '' = x'';

yields 0?

Suppose I have data in FOO table. By what means can I tell 
what is inside of my_blob column of FOO, when

SELECT length(my_blob) FROM FOO;

returns 0 for both empty string('') and empty blob (x'') alike? 
And what is the reason behind this difference?

Thanks,
Paul