With the Convolution Bloom the main thing to keep in mind here is that the kernel image you use is a whole image filter. The hottest (brightest) part of the image should be much brighter than any other part of the image by a lot! The .exr format is good for this kind of contrast. The Convolution Bloom system also expects the hottest (brightest) point of the image to be at the center, so to do this just make a white dot there and the rest of the image substantially less bright. The blurring effect you’re seeing with the other images is because they don’t adhere to this and simply won’t work well. Also for any kernel images you create if you don’t adhere to this you’ll run into the same blurring issue. I made this mistake as well when I initially started using the system.
So now that you know that, a good bloom kernel has a structure that fills most of the image kernel image you create, although if you look at the default one you may wonder why it doesn’t appear that way. In fact this kernel image (default one) uses the majority of the image! Just export it and open with Photoshop or a similar image manipulation software and adjust the contrast values. You’ll then be able to see the radial lines that come out and the spires stretch further as well. You should also look at the individual color channels as this may help with your Bloom Kernel Image construction.
All of this should be making its way into documentation soon. I don’t have an ETA when, but I just finished the addition to the Post Process Effects Bloom page for it the other day, then it’ll continue its way through our review process. 