How to define patterns to filter lists of replacement rules?

rules = {a -> 1, b -> 2, c -> 3};

FilterRules[rules, b]
(* {b -> 2} )*

or

Cases[{a -> 1, b -> 2, c -> 3}, PatternSequence[b -> _]]
(* {b -> 2} )*

Cases handles an expression with head Rule specially, providing replacement functionality, therefore you need to keep the literal head of your pattern from being Rule. Examples:

Cases[rules, _[b, _]]
Cases[rules, x : (b -> _)]
Cases[rules, HoldPattern[b -> _]]
Cases[rules, Verbatim[Rule][b, _]]

All evaluate to {b -> 2}. None of the patterns have the head Rule:

Head /@ {_[b, _], x : (b -> _), HoldPattern[b -> _], Verbatim[Rule][b, _]}

{_, Pattern, HoldPattern, Verbatim[Rule]}

Of course for this particular operation FilterRules is more direct.