I suppose that you are examining a Finale document created by someone else, right?

Even if everything prints in Black, on the computer screen Finale displays different colors for different items.

You can change Finale’s display colors if you want.

A tie always displays the same color as its Layer.

A tie in Layer 1 normally displays in Black, since Layer 1 usually is Black.

On The Other hand, a slur is a Smart Shape.

Hence all slurs normally display in red, regardless of the layer.

By "the same key" do you mean "the same staff position?" Or do you mean across an actual key change?