This was the engine for any number of scams: "blockbusting," for example, in which a realtor would convince the residents of a neighborhood that black families were about to move in, get them to sell at a discount before prices crashed, then actually bring said black families in and sell to them at outrageous prices, because they could only get special high-interest loans. Often, these loans were rigged so that they could never be successfully paid off, allowing the sellers to seize both the houses and all of the mortgage payments over the years; this is one of the major reasons why black wealth (as opposed to income) is, on the average, only one tenth of average white wealth.
But these seem like general abstractions. The scan below, unearthed by Alexis Madrigal, shows you what it looked like in practice: this is a government assessment of a tract in Oakland in 1937. It describes the tract's "favorable influences" as "convenience to local and San Francisco transportation, schools, and local shopping districts. Good climatic conditions." The "detrimental influences" are "Infiltration of colored residents. There are now about twelve families scattered over the area indicated."
If you look at the rest of the form, you'll see that under the description of "inhabitants," not only are there fields to describe common occupations and salaries in the area, but the percent foreign-born, and a field marked "Infiltration of ___". ("Orientals & Negroes," in this case) But they note that "unless one knows about the colored families living in the district, there is no means of distinguishing their homes from those of their white neighbors. The homes of the Negroes are in many instances better kept than the adjoining homes of white owners."
Nonetheless, "loans in this area should be governed according to hazard" – that is, this area was color-coded red on the map, due to the hazard of further "infiltration."
This wasn't a private organization; this was an official government form, used by a federal agency to ensure that black people could not get loans, and that any neighborhoods with black people in them would quickly become worthless from a housing perspective to anybody white: that is, to legally create ghettos.
So if you're wondering how the economics of the US, and of black poverty, came about: here's where it starts.
If you want to know more about redlining, there are a few great places to start: the Wikipedia article,
Alexis Madrigal's article about it:
and Ta-Nehisi Coates' famous "reparations" article, which is actually almost entirely about redlining as well: