  We present BLOBCAT, new source extraction software that utilises the flood
fill algorithm to detect and catalogue blobs, or islands of pixels representing
sources, in two-dimensional astronomical images. The software is designed to
process radio-wavelength images of both Stokes I intensity and linear
polarization, the latter formed through the quadrature sum of Stokes Q and U
intensities or as a byproduct of rotation measure synthesis. We discuss an
objective, automated method by which estimates of position-dependent background
root-mean-square noise may be obtained and incorporated into BLOBCAT's
analysis. We derive and implement within BLOBCAT corrections for two systematic
biases to enable the flood fill algorithm to accurately measure flux densities
for Gaussian sources. We discuss the treatment of non-Gaussian sources in light
of these corrections. We perform simulations to validate the flux density and
positional measurement performance of BLOBCAT, and we benchmark the results
against those of a standard Gaussian fitting task. We demonstrate that BLOBCAT
exhibits accurate measurement performance in total intensity and, in
particular, linear polarization. BLOBCAT is particularly suited to the analysis
of large survey data. The BLOBCAT software, supplemented with test data to
illustrate its use, is available at: http://blobcat.sourceforge.net/ .
