Trello doesn't (currently) have a way to link items directly to each other.
The approach that works for me is to use a "waiting" -label on items that are stuck. This way they can maintain the context that their lists provide. And since items can have multiple labels it doesn't conflict with any existing labels either.
Of course, if your lists don't contain any context worth maintaining, a separate list for waiting items works well too.
For updating the state of the dependent card, I suggest Bob Snodgrass's answer of adding a checklist item for each prerequisite card, that says "Add note on dependent card X when complete". Or if you need further organization, you could even create a "Dependencies" -checklist on the dependent card with items for each prerequisite card. Then people completing the prerequisite card can just go mark their card as done.
The best feature of Trello is how it keeps all the features simple - so that's probably why there's no dependency, since it would make things more complicate. That said, I think a feature to link items (if kept simple and clear) would be a wonderful addition, as it is a basic element of keeping work organized.