In HBO's Game of Thrones TV Show, the depiction of the Kingsguard has had various, minor changes throughout the show, but was always relatively consistent and faithful to the books, up until Season 7.
The general description of the Kingsguard's equipment seems to be: a long white cloak and white armour, usually with enamelled scales.
Lannister’s men were everywhere. Jaime wore the white cloak of the Kingsguard over his golden armor. I can see him still.
—A Game of Thrones, Chapter 12, Eddard
When she got closer, she saw two knights kneeling before the queen, in armor so fine and gorgeous that it made her blink. One knight wore an intricate suit of white enameled scales, brilliant as a field of newfallen snow, with silver chasings and clasps that glittered in the sun. When he removed his helm, Sansa saw that he was an old man with hair as pale as his armor, yet he seemed strong and graceful for all that. From his shoulders hung the pure white cloak of the Kingsguard.
—A Game of Thrones, Chapter 15, Sansa
The seven knights of the Kingsguard took the field, all but Jaime Lannister in scaled armor the color of milk, their cloaks as white as freshfallen snow. Ser Jaime wore the white cloak as well, but beneath it he was shining gold from head to foot, with a lion’shead helm and a golden sword.
—A Game of Thrones, Chapter 29, Sansa
“A paper shield,” the eunuch said. “Try not to look so shocked, Lord Stark. Jaime Lannister is himself a Sworn Brother of the White Swords, and we all know what his oath is worth. The days when men like Ryam Redwyne and Prince Aemon the Dragonknight wore the white cloak are gone to dust and song.
—A Game of Thrones, Chapter 30, Eddard
The closing of the door behind him silenced the voices. Ser Boros Blount was stationed outside the chamber, wearing the long white cloak and armor of the Kingsguard. He gave Ned a quick, curious glance from the corner of his eye, but asked no questions.
—A Game of Thrones, Chapter 33, Eddard
In the books, Jaime likes to differentiate himself and departs from the traditional white armour, opting for a Lannister gold armour, instead. While this is an exception, it may be relevant and important for answering the question.
The show has depicted them faithfully, with some minor changes throughout the seasons which reflected the political changes in King's Landing:
Season 1 has the most faithful versions of the Kingsguard armour, with the white frosted decorations, white cloak and the scales, even though Jaime is technically supposed to wear a distinct Lannister gold armour, they probably went with this choice to make it easier on the audience to understand he is part of a Royal bodyguard brotherhood:
Season 4 we start seeing more gold creeping in, as the white frosting was phased out, and Jaime starts to show some wine-red in the collar of his under-garment. Still faithful with the white cloak, and in-line with the original armour from Season 1 - this was probably done to subtly show the Lannister's growing boldness and control over the throne, despite the Baratheon's technically being the royal house:
Season 6 reflects the political affiliation that the crown has made with the Faith of the Seven's Sparrows, so we see that the show's version of the Kingsguard's standard (the three swords) has been replaced with the symbol of the Seven-Pointed Star representing the church's Faith of the Seven:
The Tower of Joy flashback, set decades prior to the show's beginning, during the reign of the Mad King Aerys II Targaryen, features a Kingsguard armour that is more silver, and boasts a Targaryen sigil (three-headed dragon), most likely the showrunner's attempt to differentiate these Kingsguard, and very clearly indicate who is on who's side.
Season 7 begins and we are introduced to a depiction that is nearly irreconcilable with the source material. Black armour with silver accents, including an embroidered sigil (symbol corresponds with Queen Cersei's crown) on the chest piece, and more importantly no cloaks(!!!).
Why are the Queensguard in Season 7 wearing this black armour?
I can presume a meta/out-of-universe reason (Cersei is wearing a stunning black, but in order for the creators to convey even more of an effect that she is in control, they match the Queensguard costumes with her outfit's colours), but I don't know whether it is justified in-universe, or what the reason could be.
Yes, Dark is Evil, but are we supposed to believe the Queensguard/Queen got together and decided to ditch the centuries old tradition of white cloaks (something they are even nicknamed as), and replace white/gold armour with pitch black armour, because they decided "It's more evil"? I'm looking for in-universe justification. The out-of-universe reasoning is obvious enough, it just makes almost no logical sense in-universe.