There isn't a single global setting to change every symbol outline color at once — for example, to make all Block Schematic symbol outlines black. However, it is straightforward to recolor symbol outlines in bulk using the Unlock cursor.

Either:

1. Select the Unlock Cursor; this lets you select and edit individual objects inside symbols and groups without changing the hierarchy of that symbol or group.
2. Use shift+click to select the symbol outlines you want to change.
3. Set the Line Color you want in the Properties Grid on the right of your screen.

Or, because symbol outlines normally sit on their own layer:

1. Open the Layers dialog (Ctrl+L) and switch off every layer except the Outline layer.
2. Choose the Unlock Cursor.
3. Select the outlines (use Edit › Select All, or drag a selection box) and set the line color you want.
4. Switch the other layers back on in the Layers dialog.

This technique lets you recolor every symbol outline in a drawing in a single pass rather than editing each block individually.

Important: The Unlock cursor is very powerful but can also be dangerous; be sure to return to the default cursor (hit Escape) after making changes to objects in symbols with the Unlock Cursor.