Q learning for blackjack, reward function?
It depends on the goal of the project. Projects can be a spectrum from "pure" to "applied":

A completely pure project only give minimal information. In the case of the Blackjack reward signal, that would just provide the win, loss, or tie information to the agent at the end of each hand.

A more applied project could give more information to the agent before, during, or after each hand. 

An applied project will most likely start training better, learn faster, and end training higher. However, an applied project is less likely to provide evidence about the specific capacities of the system.

For both pure and applied projects, the amount and type of reward signal is a hyperparameter that is best tuned through empirical experimentation.