cellcolor{black!##1}

documentclass{article}
usepackage[table]{xcolor}
usepackage{pgfplotstable}

pgfplotstableset{
    color cells/.style={
        col sep=comma,
        string type,
        postproc cell content/.code={%
                pgfkeysalso{@cell content=rule{0cm}{2.4ex}cellcolor{black!##1}pgfmathtruncatemacronumber{##1}ifnumnumber>50color{white}fi##1}%
                },
        columns/x/.style={
            column name={},
            postproc cell content/.code={}
        }
    }
}

begin{document}
begin{table}caption{Correlation or something}
centering
pgfplotstabletypeset[color cells]{
x,a,b,c,d
a,90,10.5,0,0
b,0,80,10,10
c,0,0,95,5
d,0,10,5,85
}
end{table}
end{document}

documentclass[border=5mm]{standalone}
usepackage{pgfplotstable,colortbl}
pgfplotsset{compat=1.12}

pgfplotstableset{
  /color cells/min/.initial=0,
  /color cells/max/.initial=1000,
  /color cells/textcolor/.initial=,
  color cells/.style={
    postproc cell content/.append code={%
          pgfkeys{/color cells/.cd,#1}%
      pgfkeysgetvalue{/pgfplots/table/@preprocessed cell content}value%
      ifxvalueempty%
      else%
        pgfmathfloatparsenumber{value}pgfmathfloattofixed{pgfmathresult}%
        letvalue=pgfmathresult%
        pgfplotscolormapaccess%
            [pgfkeysvalueof{/color cells/min}:pgfkeysvalueof{/color cells/max}]%
            {value}{pgfkeysvalueof{/pgfplots/colormap name}}%
        pgfkeysgetvalue{/pgfplots/table/@cell content}typesetvalue%
        pgfkeysgetvalue{/color cells/textcolor}textcolorvalue%
        toks0=expandafter{typesetvalue}%
        edeftemp{noexpandpgfkeyssetvalue{/pgfplots/table/@cell content}{%
            noexpandcellcolor[rgb]{pgfmathresult}%
            noexpanddefinecolor{mapped color}{rgb}{pgfmathresult}%
            ifxtextcolorvalueemptyelsenoexpandcolor{textcolorvalue}fi%
            thetoks0%
          }%
        }%
        temp%
      fi%
      }%
  }%
}%

begin{document}
pgfplotstabletypeset[col sep=comma,
/color cells/max=100,
/color cells/min=0,
/color cells/textcolor=white,
columns/a/.style={color cells},
columns/b/.style={color cells},
columns/c/.style={color cells},
columns/d/.style={color cells},
columns/x/.style={string type},
/pgfplots/colormap/blackwhite,
]{
x,a,b,c,d
a,90,10.5,0,0
b,0,80,10,10
c,0,0,95,5
d,0,10,5,85
}
end{document}