In this line, "But my late spring no bud or blossom shew'th" two metaphors. The first one is of the "bud" which refers to the early signs of greater achievement. The "blossom" stands for an accomplishment. Besides, Milton says he is in his "late spring". As he has not accomplished his goal in his youth, he thinks he might be nearing the late springs of his youth. So, through this line, he says that he has not achieved any feats that distinctly mark his intellectual prowess and youthful vigor.