1st off I`ve no experiance with Power Pistol in the 38.
I do however, use it in the 45acp for heavy loads in place of Unique. Power Pistol is a little slow for what I believe you are trying to do. I would stay with the Bullseye load, it will work in either pistol or switch to jacketed bullet if you want more velocity. My dad used lots of 140 gr Speer jacketed bullets in both his 38spcl and 357 with Unique powder stuffed in 38 cases when he was still shooting. The load escapes me but it wasn`t a top load but some where in the 850-900fps range as the book reported. (lite to mid-level) Top velocity loads for the 357 were loaded with 2400 which is slower yet then Power Pistol.
I agree with the poster that suggested a lot of what you are likely seeing is bullet lube burning. Power Pistol also burns cleaner as pressures rise in the 45acp and likely do the same in the 38. Both cartridges are low pressure rounds to begin with and until you get in the pressure range of the 357/9MM you will likely have some soot associated with it also.