Unfortunately, I think you are speaking about what people commonly say is "Huygen's Principle", "In order to explain waves diffraction, it says that every point in a wave front behaves as a source, so the next wave front is the sum of all secondary waves produced by these points.", but this is not actually what Huygen's principle says.
Huygen's principle has to do with the propagation of light, which is electromagnetic waves, governed by Maxwell's equations. It can be shown that upon decoupling Maxwell's equations, one obtains spacetime wave equations of the form:
$u_{,t,t} = c^2 \left(u_{x,x} + u_{y,y} + u_{z,z}\right)$, (commas indicate partial derivatives) subject to the boundary conditions:
$u(\mathbf{x},0) = u(\mathbf{x}), \quad u_{,t}(\mathbf{x},0) = \psi(\mathbf{x})$.
The solution is given by D'Alembert's formula, but in the context of space-time wave equations, is known as Kirchhoff's formula or the Poisson formula, but it is the generalization of the Huygen-Fresnel equation, and is given by:
$$u(\mathbf{x},t_{0}) = \frac{1}{4\pi c^2 t_{0}} \iint_{S} \psi(\mathbf{x})dS + \left[\frac{1}{4 \pi c^2 t_{0}} \iint_{S} \phi(\mathbf{x}) dS\right]_{,t_{0}}.$$
You see from the solution that the point of Huygen's principle is to ensure causality of wave propagation. That is, as can be seen from the solution that $u(\mathbf{x}_{0},t_{0})$ depends on the boundary conditions on the spherical surface $S = \{ |\mathbf{x}-\mathbf{x}_{0}| = c t_{0} \}$, but not on the values inside the sphere! That is, the boundary conditions influence the solution only on the spherical surface $S$ of the light cone that is produced from this point.
This is precisely Huygen's principle: Any solution of the spacetime wave equation travels at exactly the speed of light $c$. So, as you can see Huygen's principle is independent of any specific slit/aperture configuration, it will apply in any situation where you can set up such boundary conditions for the spacetime wave equation!
Best Answer
This is similar to the reasons one way mirrors work.
If you look through a black net then no light is reflected from the net so the eye sees only the light coming from the objects on the far side of the net. The amount of the external light that reaches you is reduced, but the brain is pretty good at reconstructing images from only partial data, so the view looks unchanged.
If you look through a white net then the eye receives a mixture of the light reflected from the net and the light from outside transmitted through it. If the room you are in is dark and the outside is bright, then the amount of light reflected from the net is small compared to the transmitted light and you still don't see the net. However if the room is light and the outside dark then the light reflected from the net swamps the light transmitted and you only see the net. In between you'll see both the net and the view.