Blob tools usually work on binary images that are created using thresholding techniques. Objects can be any shape and complexity and can also contain any number of holes which can also be measured as objects if required.
The following images show a blob tool used to differentiate between a collection of electrical components. The input image is on the left, and to the right is the binarised image. Image processing has been used to show the centroid of each detected object for easy identification. There are many parameters that can be measured for each object such as area, perimeter, centroid, number of blobs or holes, plus first and second order moments.