Beans sold for espresso are coffee selected or blended with that preparation in mind. They can include Arabica, Robusta or a combination, and the roast may be dark or lighter depending on the roaster's intention. There is no separate botanical espresso bean.
Read the bag's flavor description and suggested use rather than assuming that darker always means more suitable. If you mostly make milk drinks, consider the balance of the complete drink. If you drink espresso alone, pay attention to the profile you actually enjoy without additions.
Grind size must be suited to the machine and basket. Follow the equipment instructions and begin with a measured dose and output. Avoid using a generic claim about all steam-driven or pump-driven machines as a substitute for the exact manual. Changing beans usually requires another adjustment of the recipe; the espresso label does not eliminate that work or guarantee a fixed extraction time.
The conversation
A good question can make a better brew.