Something to consider before useing exhaust wrap is the moisture barrier it creates between it and the pipe. On cool down moisture will accumilate between it and the pipe, and eventually will cause the pipe to rust out. Its should'nt cause an exhaust because of the double wall pipe, but would on single wall pipes.

I can see where the exhaust wrap would make the outer pipe turn blue. That double wall is designed to allow the outer pipe to be cooler, by allowing the outside air to cool it. The wrap would defeat the purpose of the double wall. The double wall is, itself, a wrap.