Why is there an annoying black rule around print area...

It looks as if you (accidentally) clicked a border color while that text frame was selected. Easy enough to do, and fix.

Good place to note one of ID's less obvious features: if you have no object selected and you click on almost any setting (color, stroke, font, etc.), it will become the default for all new objects to which it applies. That is, if you had no object selected and clicked a black outline color, or set a stroke width, everything you created after that point would have a black stroke applied. The fix is to do the reverse: with no object selected, click on the desired default instead.

Not saying the latter is your problem here (more likely the first, as everyone has noted), but it's a quirk worth  knowing when, say, everything is coming up with a blue border.

As Willi states, you may have an Object Style applied as a default – you can see in the screenshot there's a 1pt border being applied.

Does your text frame or its object style have a stroke defined. Click on the frame and have a look. If nothing is selected click on no stroke in the tool panel.