This isn't the easiest to understand algorithm in the world but I think it does exactly what you want.
n = size(C,1)
ix = ceil(mod((1:n)/10, ceil(n/10)))';
accumarray([[ix;ix], [ones(n,1);ones(n,1)+1]],C(:),[],@mean)
ix is ten 1s, then ten 2s, etc. up until 8 21s.
This is replicated twice in accumarray for each column of C. The second column of subs is 208 1s followed by 208 2s indicating first and second column. vals is all valuex of C.
