What is the default ColorFunction for MatrixPlot?

OK, this turns out to be version related. Based on the experience from this post:

In v9.0.1 or earlier

After some trial and error, I obtain:

colorFunctionMatrixPlot = 
    Trace[MatrixPlot@{{1}}, _[Blend[__]] &] // Flatten // First // ReleaseHold

Quiet[Blend[{{0., RGBColor[0.260487, 0.136843, 0.891569]}, {0.166667, 
     RGBColor[0.230198, 0.499962, 0.848188]}, {0.333333, 
     RGBColor[0.392401, 0.658762, 0.797589]}, {0.499999, 
     RGBColor[0.648371, 0.823316, 0.898802]}, {0.5, GrayLevel[1]}, {0.500001, 
     RGBColor[0.857832, 0.857603, 0.654231]}, {0.666667, 
     RGBColor[0.913252, 0.790646, 0.462837]}, {0.833333, 
     RGBColor[0.860243, 0.558831, 0.00695811]}, {1, RGBColor[1., 0.43125, 0.]}}, #1]] &

MatrixPlot[{{1, 2, 1}, {3, 0, 1}, {0, 0, -1}}, ColorFunction -> colorFunctionMatrixPlot]

In v10

colorFunctionMatrixPlot = 
    Trace[MatrixPlot@{{1}}, Blend[__]&] // Flatten // First // ReleaseHold;
colorFunctionMatrixPlot[[1, 1]] = colorFunctionMatrixPlot[[1, 1]];
colorFunctionMatrixPlot

Blend[{{0., RGBColor[0.260487, 0.356, 0.891569]}, {0.166667, 
    RGBColor[0.230198, 0.499962, 0.848188]}, {0.333333, 
    RGBColor[0.392401, 0.658762, 0.797589]}, {0.499999, 
    RGBColor[0.964837, 0.982332, 0.98988]}, {0.5, RGBColor[1, 1, 1]}, {0.500001, 
    RGBColor[0.95735, 0.957281, 0.896269]}, {0.666667, 
    RGBColor[0.913252, 0.790646, 0.462837]}, {0.833333, 
    RGBColor[0.860243, 0.558831, 0.00695811]}, {1., RGBColor[1., 0.42, 0.]}}, #1] &

Notice the color scheme has been changed a bit. See the comments below for more details.

We can get the default color function for MatrixPlot using Themes`DefaultStyles:

dcf = "DefaultColorFunction" /. Themes`DefaultStyles[MatrixPlot][[All, 2]]

 Blend[System`PlotThemeDump`$ThemeDefaultMatrix, #1] &

System`PlotThemeDump`$ThemeDefaultMatrix

MatrixPlot[{{1, 2, 1}, {3, 0, 1}, {0, 0, -1}}, ColorFunction -> dcf]