After trying a lot of (complex) stuff like reimplementing old KonamiBlendCopy from the old fba-libretro repository, without success, i tried simply changing :
PutPix(pBurnDraw + (i * nBurnBpp), BurnHighCol(bmp[i]>>16, (bmp[i]>>8)&0xff, bmp[i]&0xff, 0));
to
PutPix(pBurnDraw + (i * nBurnBpp), BurnHighCol(bmp[i]>>16, bmp[i]>>8, bmp[i], 0));
in KonamiBlendCopy. For whatever reason, it seems it did the trick. Since i'm still kinda a noob at coding C/C++ and emulators, anyone can explain what those "&0xff" are for ?