The Radbaz (Resp. I, 296) discusses the berakhah recited on a king, and whether that berakhah would be applied to any other governmental minister. In that context he cites as a principle that the subject of the berakhah must have the authority to kill, and that authority must not be subject to being overruled by any other official. Such a principle might cast doubt on the eligibility of modern heads of state, who generally do not have unchecked ability to execute. However, R. Shmuel Wosner (Resp. Shevet HaLevi, I, 35) explains that this principle is only for determining which other officials in a government have a status similar to the king, to merit a berakhah. The highest authority in the land is always of king status, even if the laws of the land do not permit him to kill. He further notes that the berakhah applies to kings and queens equally, and does not require actually seeing the monarch.