Ad Reinhardt (b. 1913-1967) is often associated with the Abstract Expressionist movement of New York, but like Barnett Newman, his work often defies this categorisation, and is now considered a big influence on minimal art and monochrome painting.
He is perhaps best known for his so-called ‘black paintings’. At first, these works seem to be black monochromatic paintings. However, if one stays with the work long enough, their eyes will adjust to and reveal the variance in colour and formal elements (many of these take on the form of a crucifix or a grid). Earlier versions required just a few minutes of the viewer for their eyes to adjust, but my understanding is that as he advanced in his career he produced paintings that would require twenty or thirty minutes of viewing.
There is something incredibly Zen about this series of work. They are an opportunity to be present with something we think we know, and over an extended period of time (a meditation?) come to learn how different things really are beneath the surface. In the process, we become intimate with the work and revel in its secrets.