A blob is a binary linked object, a contiguous mass of pixels on one side of a threshold, surrounded by the other polarity. It is not a grayscale tool. Threshold decides the tipping point. Raising it turns more gray into black.
The tool can report count, area, centroid, angle, bounding rectangle, holes, perimeter, elongation, and spread. The outline can become a region for a child histogram, filter, or another blob. A sort can order a tray by position, grid, distance, or angle so later logic can address item 7 rather than “whatever was largest.” Bounding rectangles are often better character centers than weighted centroids, because letters such as T or L are top- or corner-heavy. Boundary handling, hole filling, and area limits are the usual reasons a blob disappears while the part is still in the picture. Blobs are a poor choice when the job needs fine pose, and a strong choice for debris, bands, presence, and rough character location.