What does black mean?

Hi, I had another newbie question:
I’ve been playing around with tutorial 1, and just for kicks I set

deltaRho = 0;

and instead instantiated the box centralSquare with an initial velocity

Array<T,2> u1(1,0);

The initialization of centralSquare then looks like

lattice, centralSquare, rho0+deltaRho, u1);

To start with, this did pretty much what I would expect - a square with a certain velocity shows up in the first image, but is quickly quenched by the surrounding fluid. However, after a while a shroud of blackness seems to originate at the position of centralSquare, which quickly engulfs the entire lattice. I know what red and blue and so on means, but what exactly does black signify? I’d expect it simply shows that something crashed in the calculations, but I’d be very happy to find out more specifically how to interpret it!