Fran, if you notice you have shades of the background purple reflecting off the surface of your blue blobby object. You must set the fuzziness to a lower or a higher number to try and isolate just that shade of the background purple. The default is 32. The higher this number the wider the shade of the foreground color is selected (foreground color decides the color to be selected). Blue and purple are very close in the color spectrum so you'er getting the bleed over effect.

Sometimes it's best to use the magic wand tool. check off anti-alias and contiguious and in this case set your tolerance to about 20. With the wand tool click on the background color (purple). Play with the tolerance number to get as close as you can. When you notice you have your marching ants generally where you want them take a good close look at the blob's reflections and you'll see that some of them are also selected. Press "Q" to turn on Quickmask and press "B" to select the brush tool. Make sure you have the pencil tool selected (nice hard edge brush). Press "D" to select the default black/white color palette with black being your foreground color and dab in those reflections (make sure your brush is set at 100% opacity and "normal" mode). You can adjust the brush size by pressing either bracket "[" "]". If part of the backround is selected then press "X" to swap the palette foreground color to white and dab to de-select the background. Continue ulternating the quickmask mode with the marching ants by pressing "Q". Patience is a virtue in this case.

There are many other tricks like turning the image to black/white using the treshold feature and adjusting the slides to the best fit and then painting in where it's needed. This then becomes the selection mask for the color image.