The ornamental design for a string of blocks, as shown and described.
FIG. 1 is a perspective view of a string of blocks showing our new design with the blocks in a first orientation;
FIG. 2 is a front elevational view thereof, the rear being the same;
FIG. 3 is a top view thereof, the bottom view being the same;
FIG. 4 is a left end view thereof;
FIG. 5 is an right end view thereof when there is an even number of blocks; and,
FIG. 6 is a front elevational view like FIG. 2 showing the string of blocks mounted vertically.
The broken lines showing the finials in FIGS. 1-6, a third block in FIGS. 1-3 and 6, square surface detail in FIGS. 1, 2 and 6 and mounting block in FIG. 6 are for illustrative purposes only and form no part of the claimed design.