One way to make it darker is to change the Bias value to below .5. In the examples I showed, I only demonstrated raising the bias above .5.

The Bias node is mathematically identical to a Pow(er) function. It differs only in that the exponent is expressed in a more convenient way.

Mathematically, Bias is defined like this:

Bias(a, b) = Pow(a,  ln(b) / ln(.5))

where ln is natural logarithm.

[ Curious note for math-heads: Strangely, the Bias function is commutative, i.e. Bias(a, b) = Bias(b, a) ]

Anyway, the way I demonstrated that shader was:
Blender(1, Bias(AO, b), d) where b is .5 to .7 and d is 1 or less.

The b value selects an exponent such that Bias(.5, b) = b. In other words, given the input value is 1/2, the output value is b. The rest of the curve is constrained to be an exponential curve that passes through the points [0, 0], [.5, b], [1, 1].

When b is .5, it means the curve is a straight line from 0 to 1. When b is less than .5 the curve bends downward, and when b is greater than .5 the curve bends upward.

When the curve bends upward, the visual effect is that the shadow is brighter (less dark). When the curve bends downard, the visual effect is that the shadow is darker.

In either case, the Blender is blending the result with white. That means that as you decrease the blending value, d, below 1, you get more of that mixed with white. This effectively changes the value when the Bias produces a 0 to something higher than 0.

So, to tie it altogether, d selects the maximum darkening (0 means no darkening at all, 1 means darken all the way to black) and the bias, b, bends the curve between the darkest and lightest values. 

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)