Is butter a heterogeneous mixture

When you melt butter, you'll find milk solids floating on the surface. This means that the mixture itself has indeed been separated, heterogenization, that is - it has lost it's uniformity. A heterogeneous mixture is one that has phases in which the particles can be separated by physical means - heat being one of them. By appearance, butter does seem to be a homogeneous mixture - when you spread it, it is indeed uniform throughout. But because it's particles can be physically segregated, it is otherwise a homogeneous mixture.

So yes, because it's particles can be separated physically, it is classed as a heterogeneous mixture.

Blood is also a good example. Seemingly homogeneous - but when you place it in a centrifuge, it separates into the red blood cells, white blood cells, platelets and the blood plasma.

Hope this clears up your question about heterogeneous mixtures