You want to look to the word raza in its usage before, during, and after 1492. Typically it was used in a very loose way before 1492, meaning other cultures, rather than other skin colours. During 1492, when the Catholic monarchs have taken the last stronghold of Muslims Spain, they utilise the concept of limpieza de sangre (blood purity) for the forcibly converted Muslims and Jews, called moriscos and conversos. They and their descendants were seen as suspicious, potential enemies even after conversion, so any Moorish or Jewish blood was enough to find yourself surveilled and suspected by the Inquisition. This is where we get the "one drop" rule of racism from, I believe. That was in 1492, in January, when Granada and Cordoba fell to the Catholic pogrom. In that same year, the same monarchs gave Columbus his chance to go westward toward the famed East where spices, fabrics and treasure awaited. Of course he bumped in the Americas quite by accident, but upon finding a people who appeared to have no comparable religion (unlike Jews and Muslims with their known written texts and long term proximity to Christians) they took that religious racism and developed a new kind of biological racism... are these people even really human? was a legitimate question back then, as they proceeded to wipe Taino and Carib indios out with slavery, murder, rape and disease. During this very same period of Spain's and Portugal's political marriage and conquest of the Iberian Peninsular and discovery of the so-called "New World", Portuguese ships were nearly rounding the Southern coast of Africa, and did so by 1495, once again in order to breach into the trade that was taking place in the east in centres like Egypt, India, Syria, Persia, China and elsewhere, including in Bantu Africa. We know there were already discourses focused on the blackness of Africans, and more so on nearer neighbours like "Saracens", "Moors" (North Africans), "Turks", "Arabs" etc, all sometimes even interchangeable names for the enemies during the Crusades, but it was the usage of enslaved Africans to replenish the obliterated workforce of the enslaved Native Americans that began the large-scale application of the religious-turned-biological racism against Africans. You have to deny someone's humanity if you are going to enslave them. Like with the Native Americans, Africans were constructed in the European imagination to be people without true religion, without religious texts, and therefore perhaps not really even human. Jews and Muslims were seen as humans but barbarians who could be, not without defects, converted.

The way European power over the world soared from the point of the discovery of the Americas and the resulting scramble by European powers in competition over new colonies also propelled racism into the full force it would become over the last 500 years. It was a circular logic, defeated people were inferior, inferior people were defeated. Enslaved people were animals, animals were enslaved. Spain and Portugal would be overtaken by the Dutch, then the French, then the English on the high seas of colonisation, with each European power bringing their own flavour to racism and with the booming European economy, literary output and philosophical/scientific endeavours creating new opportunities to for Europeans to show just how prejudiced they could be... so as religious racism would transmute to biological, biological would transmute to 'scientific' racism where the biological inferiorities of non-European races (or even less European races) would be demonstrated via crude comparisons of skull shapes and other such nonsense. Anthropology, travel literature, colonial records, philology and philosophy textbooks would be filled with wisdom about the way the world is according to European imagination. And all the meanwhile, in this Enlightened atmosphere, the cruellest violence was still being carried out en masse in the colonies, plantations, mills and sweatshops that made the new capitalist era function. All of that, all those contradictions, requires continuing racism to make it make sense.

We are still dealing with the aftermath and the legacy which is not going away quickly. That is why today, like you say, we talk about power imbalance. These racist histories lead to a power imbalance, and power imbalances sustains racist history. They are two sides to a coin. One can have antipathy towards some of an opposite race, but when we are talking about racism in academic discourse today we are wanting to draw attention to the fact that it is more than just prejudice and preferences, but is a function and foundation of much of the material realities we have today from those 500 years of massive exploitation of some for the profit of others.

Hi, if you don't get answers here, you might try x-posting to /r/AskSocialScience