I got a genuine nokia dku-5 cable which have the same cable color code as yours. Since I don't have a multimeter I tried your suggestion and connected blue and red as well as the black cable as mentioned.
I started the terminal and got scrambled characters displayed like you. At first i thought that there is a problem with the baud rate but even trying any baud rate available could not solve this issue. Depending on the baud rate there is scrambled or no display at all.
Following this, I think your color combination could be either wrong or different. I don't know. The strange thing is that after reversing the blue and red cable i still got scrambled output. If blue and red is really a working Rx & Tx then this should not happen.
I was careless enough to cut the cable in such a way which made it impossible to reconnect the pin plug to the usb converter in order to access the right connection through the original pin plug I also destroyed the pin plug during the attempt to open the plug in order to see to which pin the colors are leading. In fact it is impossible to access the inside of the pin plug because every thing is infused in some kind of resin.
Rx & Tx detection through loopback failed as well. There is no combination which loops the signal back.
Maybe the converter needs 3,3 V power supply in order to working properly but the question is still the same... where? Even if the red/blue combination is right and black is ground then there is still a white and a yellow cable.
I spend hours searching for color code informations on the specific genuine dku-5 but everybody seems to connect with the cloned ones.