We present in this work a generic and effective method to increase the prediction accuracy of no-reference image/video blur assessment facing the real-world content diversity. We demonstrate that benchmarking no reference image blur metrics, fitting a single logistic function to map the objective predictions to subjective scores in the well-known databases like LIVE or TID2008/2013, introduce biased fitting results towards better predictions only in the central part of the score scale. We find out that a multi-fitting approach, using the correlation parameters between subjective scores and objective predictions for content clustering and then conducting logistic fitting for each content type, can evidently improve the metric prediction accuracy in the full score scale. Besides, the overall prediction variance is also reduced with the proposed scheme, presenting more consistent results insensitive of content variation. We prove that the proposed method is of practical meaning to facilitate blur assessment techniques validated on limited databases to the vastly abundant real-life content types.