There are two dark moments around a blackhole and their boundaries are at two different radii.

The first is the obvious escape velocity radius of no return.

If given the equation v_e = √2·G·m / r_e, where v_e is the velocity and r_e is the distance from the center of blackhole mass, m, then we can set v_e equal to the speed of light, c.

The equation becomes c = √2·G·m / r_e; solve for r_e it becomes, r_e = 2·G·m / c².

The second might not be so obvious; it's the centripetal velocity radius of no return.

This is a balance between two forces; one centripetal: m_o·v_c² / r_c and two gravitation: -G·m·m_o /  r_c², where m_o is the mass of smaller object, m is the mass of the blackhole, v_c is the centripetal velocity around the blackhole at radius r_c.

The second equation becomes (m_o·v_c² / r_c) - (G·m·m_o / r_c²) = 0, solve for r_c it becomes, r_c = G·m / v_c².

Now set v_c equal to the speed of light, c, it becomes r_c = G·m / c².

As we can see, the two boundaries are different by only a factor of 2, r_e = 2·G·m / c² and r_c = G·m / c², but the velocities are at 90 degrees of each other.

The escape velocity is pointing out from the center of the blackhole, normal or perpendicular to the circle swept out by the radius r_e.

The centripetal velocity is pointing on a tangent to the circle swept out by the radius r_c.