The Spanish navy had an essentially Mediterranean outlook on naval warfare. Following the operational doctrine which had held sway in the Med for 2000 years, they considered ships to be platforms from which infantry would fight. Their object was to ram or lay along side a ship, and board the ship to defeat the soldiers on that ship, and capture or set fire to it. To that extent, yes, they relied heavily on row galleys. Their operational doctrine was so deeply embedded in the psyche of the navy that the guns on their ships were huge, long-barreled guns on large carriages, which were designed to be re-mounted on different carriages for use on land when the Armada reached the Low Countries. This significantly hampered their ability to reply to the fire of the English ships, which, although they used long-barreled guns, used guns of the naval type, which had significantly shorter barrels, and which were mounted on dedicated naval carriages. The Spanish guns had to be withdrawn and turned sideways to be reloaded. The English guns were simply rolled back, reloaded and rolled forward to fire again. A good rate of fire for the English ships was one broadside in between two and three minutes. The Spanish were lucky to fire a gun once every eight or nine minutes. The Spanish expected to fire the guns once at close range, and then lay along side to board. Their doctrine did not even provide for reloading their guns, and the tackle was designed to prevent the guns from recoiling--the English and Dutch guns were designed to recoil within the range of the tackle, so that they could be quickly reloaded. Naval archaeologists who have dived on the Spanish ships have found that most of their ammunition was unused in the battle at Gravelines, while the English and Dutch ships were forced to withdraw when they ran out of powder--having done terrible damage to the Armada.
However, all the evidence of which i know states that the Armada was the first attempt of the Spanish to engage in a major fleet action with sailing vessels. They employed more than 20 galleons, and more than 100 converted merchantmen. They did, however, have many smaller ships which were used as "packets," which is to say, ships which deliver messages or transfer crew and supplies from one ship to another. Therefore, a good many galleys did comprise the Armada, but they were smaller vessels, used as packets, and not the large warship row galleys used in traditional Mediterranean warfare at sea. Some of the galleys to which i have read of references were galleasses, which were heavier row galleys, designed for the ramming an boarding technique. I know that several were used by the Spanish at the battle off the Isle of Wight, where the wind was against them--the galleasses were used to tow the galleons into a position from which to fight the English with naval artillery.
This source at the BBC mentions 2080 galley slaves. It is entirely reasonable to assume that any number of them were Africans. Some may have been Berbers and Moroccans captured in war, and some may have been black African slaves purchased from the Muslims, who had a slave trade for centuries before Europeans became interested in the slave trade.
This source, from the Guardian newspaper is part of a set of lessons for elementary school children, and it lists galleasses and galleys as a part of the Armada. Whether or not there were African slaves on the galleys, i could not say, although it is highly likely. In Mediterranean fleets, such as the Spanish and the French fleets, galleys were often powered by galley slaves who were condemned criminals--however, as those who were condemned to the galleys were those who had been sentenced to death, and whose sentences were commuted to a life sentence on as a galley slave, it is doubtful that they could rely exclusively on that sources. Additionally, the Spanish had recently been fighting on the coast of Africa, and the islands off the coast. The King of Portugal had been killed in battle in 1578 without an heir, and in 1580, Philip II of Spain, because his mother was Portuguese, claimed the throne and invaded Portugal. In succeeding years, the Spanish sent expeditions to seize the Portuguese possession on the west African coast, and the important islands off the coast. It is completely plausible that they picked up Africans for use as galley slaves during those campaigns, less than a decade before the Armada sailed for England.
Short answer: can't say for certain, but it is highly probable.