since you already have the location and the size, you can sample the mean from a rectangular crop, centered around the keypoint position:
Point p = keypoints[i].pt;
int w = keypoints[i].size;
Rect r(p.x-w/2, p.y-w/2, w, w);
Scalar m,d; // mean and stddev
meanStdDev( img(r), m, d);