In strings (or Unicode objects in Python 2), \u has a special meaning, namely saying, “here comes a Unicode character specified by it’s Unicode ID”. Hence u"\u0432" will result in the character в.
The b'' prefix tells you this is a sequence of 8-bit bytes, and bytes object has no Unicode characters, so the \u code has no special meaning. Hence, b"\u0432" is just the sequence of the bytes \,u,0,4,3 and 2.
Essentially you have an 8-bit string containing not a Unicode character, but the specification of a Unicode character.
You can convert this specification using the unicode escape encoder.
>>> c.decode('unicode_escape')
'в'