Blind digital watermarking, which can detect watermark without using the original image, is a key technique for practical intellectual property protecting systems and concealment correspondence systems. In this paper, a blind detection method for the digital image watermark is discussed. The theoretical research shows that the orthogonal projection sequence of a digital image is one-to-one correspondence with this digital image. By this conclusion, a kind of blind watermarking detector with good performance is designed and realized. To calculate the correlation value between the image and watermark, the pixel information of digital image is not adopted, but the orthogonal projection sequence of this image is adopted. Experimental results show that this watermark detector has not only the good robustness to Gaussian noise, but also the very strong resistant ability to translation and rotation attacks. Performance of this watermark detector is better than that of the general detector designed by using the pixel value information directly. The conclusions are useful for the research in the future.