How can i darken outline of brushes in CS6

You can't. AFAIK

I am not sure whether this is what you have i mind but it may serve your purpose. Unsharp masks create their effect by darkening the darker to two sharp adjacent tones and lightening the lighter of the two adjacent tones. So, in essence, pushing the effect beyond normal criteria, you can create and control the edge width and darkening of a line such as the brush stroke one shown here.I overdid the effect to make it more apparent.

If the stroke is on its own transparent layer you could also use layer effects such as inner glow/inner shadow and stroke.