To set the Color of a Line in a Lineseries, we are supposed to set

 lineSeries.BorderColor = Color.Black;

But if I have Labels Enabled

(lineSeries.ShowLabels = true)

the labels get a Border, just by setting the Color of the LineSeries itself.

Screenshot attached.

I just want a single line black on white Chart.

Yes it works, thank you.

This inheritance might be useful for something like a Font, for the LineColor this inheritance is never wanted. I consider this a workaround.

However, a related Question, when I draw the line on top of something else, I have a RangeSeries here,

the color seems to be modified or combined, or there might be some transparency added, or it's the AntiAliasing maybe.

How come my black line appears gray, when drawn on another background ?

Well, in reverse order, the line is invisible/completly hidden, this is conclusive.

But the Color of the Line ist set to black, and then drawn on this gray 'background' the line is not black anymore.

You can compare the axis-Line is in correct black, the Lineseries is not.

If this is not done by purpose, maybe Telerik added some automatic transparency features, if ChartSeries cover each other,

than it must be some AntiAliasing effect.

By having a look into the source-code, I found out the Lineseries is drawn as a Series of Rectangles, 

that's why we have to set the BorderColor, and maybe DrawRectangle has some different Antialiasing-Behaviour than DrawLine.

However, this is not so important, more a curiosity.

You really put a lot of effort in this, thank you.

Well, I found out, the series use reverse order, they are painted from Last to First.

A programmer always paints the background first, and everything else on top.

But when adding series to the chart, I have to Add the "background" series Last.
That's against my intuition.
And also against yours, since you wrote:
"you should add the line series after the range and the line will be painted over the area"
That's wrong. Your screenshots are the proof.

Second thing:
I hadn't set the color of the RangeSeries,
As you confirmed just now, and what I explicitly asked for, is "the default colors are transparent".

(this is not so simple to find out, since the Default Colors are applied later, at no point in your code, you can set a breakpoint and just inspect Series.BackColor, it's always Color.Transparent)

So what I had, was my black line, behind a (coincidently) grey area.

With all Colors being transparent by default,

the order of Adding things to the SeriesCollection, has actually only little influence to the result. You always see everything.

So the counter-intuitive Adding Order is just pointing out, when handling colors manually.

We finally clearified this, maybe you find a good place to document this.