Without getting too much into the present day so as not to break the rules, the idea that Europe doesn't have similar concepts of black/white is very flawed - after all, the Americas in question were not just colonized by Europeans (the Spanish, Portuguese, English, Dutch, French, etc), but also most of Africa itself.

There I came across the history of racism as a partly socially constructed phenomenon that did not exist prior to the 18th century

This is certainly not true. Race was definitely socially constructed, but it was solidified as a systemic framework around 1650 by the Spanish and English in the Americas when they began to racially define themselves as 'white' to go along with other categories such as black & Indian. Race-based discrimination against black Africans in particular was much older: depending on which scholar you're reading, it either dates back to proto-racism among slave-holding Arabs or to the Spanish colonial shift from Indigenous to mass African slavery in the early to mid 1500s. Scientific racism was developed in the 18th and especially 19th centuries, but this was a post-hoc attempt to justify already deeply ingrained ideas & social systems rather than the beginning of racism itself.

Perhaps it's more accurate to say that it became widespread in Europe itself in the 18th century, though it was already pervasive in many of the colonies that existed at the time. In David Brion Davis' history of slavery, he notes a few examples of how some very, very important figures in Europe were already extolling the superiority of 'whites' over 'blacks' at this time.

David Hume in 1748:

I am apt to suspect the Negroes, and in general all other species of men, to be naturally inferior to the whites. There never was any civilized nation of any other complection [sic] than white. . . . No ingenious manufactures among them, no arts, no sciences. . . . Such a uniform and constant difference could not happen, in so many countries and ages, if nature had not made an original distinction between these breeds of men (from “Essays: Moral, Political and Literary”).

Voltaire in 1756:

Their round eyes, their flat nose, their lips which are always thick, their differently shaped ears, the wool on their head, the measure even of their intelligence establishes between them and other species of men prodigious differences (from “Essai sur les moeurs”)

Immanuel Kant (A German!), 1764:

The Negroes of Africa have received from nature no intelligence that rises above the foolish. The difference between the two races is thus a substantial one: it appears to be just as great in respect to the faculties of the mind as in color. . . . Hume invites anyone to quote a single example of a Negro who has exhibited talents (from “Observations on the Feeling of the Beautiful and the Sublime”).

Such attitudes were definitely influenced by biased communication disseminating from the colonies, which was overwhelmingly produced by people invested in slave societies who had clear incentives to present black Africans as 'inferior'. But Europe itself was already intimately involved in the slave trade & obviously profiting hugely from slaves in their colonies. Germany, as far as I know, didn't have a slave port itself, but close to its borders France and the Netherlands had multiple that dealt almost exclusively in 'black' Africans. Many Germans & German ships were however active participants in the slave trade, and Germany's relative lack of participation is more easily explained by its inability to secure colonies at this time rather than moral superiority.

In the late 1800s/early 1900s, when Germany finally began colonizing South-West Africa, they had long since developed ideas of 'white' vs 'black' just like other European colonial powers had and referred to their victims in those terms. This colonial endeavour resulted in the Herero & Nama genocide where 40,000+ Indigenous people, considered 'black' by the colonizers, were massacred or starved to death for resisting colonization.

The reason why the concept of 'race' might bring you disgust today is due to the post-war German education system's coverage of history, which rightfully is quite focused on instilling disdain for Nazism in the population, and the Nazis obviously made ample use of racial frameworks. And while race may not have that much overt currency in Germany, it certainly retained it throughout Europe. But I think it's important to understand that the focus on 'race' in the USA, at least the 'white' vs 'black' paradigm, shouldn't be taken as a clear negative that Europeans are above.

It's not that Americans are uniquely obsessed with such distinctions, it's more that in the USA, the black minority has come to form a very strong in-group identity as a result of them being stripped of their previous identities by slavery & a shared history of oppression. The maintenance of these distinctions has evolved from a simple social construct designed to oppress black people into an identity for those who were branded as such.

Such a unified identity didn't solidify among black people in Europe because there was never such a large minority unified in that same way, so African diaspora populations there are more likely to identify with their ethnic or national background than race. Race and racism still absolutely exist in Europe and I think it's unfair to frame it as any less insidious than it in the US just because the black populations are not quite as visible or unified in their identity. In fact, the civic nationalisms in some European countries - I have France in mind here mostly - are very assimilationist in a way that fosters the erasure of such identities and encourages that very much still existing racism be swept under the rug, which creates the very illusion that race doesn't matter there.

Think about it this way: the victims of US racial oppression live in the US itself, so they're more immediately visible when you take a look at the country, its society & politics, etc. The victims of European racial oppression mostly live in their colonies or former colonies, so they're much less so.

Two books that cover the development of the 'white/black' race framework both in the colonies & Europe itself:

Inhuman Bondage: The Rise and Fall of Slavery in the New World, David Brion Davis

Racism: A Short History, George Fredrickson

While it does not directly address some of what you're asking, you can find an answer by /u/mimicofmodes related specifically to Irish and Italian whiteness here.

There is also this response from /u/TheThomasPreacher in the same question thread.

Some great information in this post by /u/petite-acorn as well as the follow-up responses.