I finally found some Shaeffer ink. I wanted blue black but all they had was (Slovenian) Blue and a botle of black. Does anyone have a recipe from which I can make blueblack with Shaeffer Blue and Shaeffer Black?
Barry

Your best bet would be to experiment with ratios and find your ideal color, as opposed to confining yourself to a formula. From what I gather, it should only take a small amount of black relative to blue to achieve the desired effect, so I'd start low and work your way up from there (maybe a drop of black for every CC of blue?).

Start of with 1 part black to 20 parts blue,
then try 1 part black to 15 parts blue
then try 1 part black to 10 parts blue.

In fact, you would do it the other way.

Put 10 drops of blue in a small (tiny) container with 1 drop black. Gives you 10:1 Blue:Black
Dip a fountain pen nib and try it.
Put another 5 drops of blue. Gives you 15:1 Blue:Black
Clean your fountain pen nib, dip it and try that.
Put another 5 drops of blue. Gives you 20:1 Blue:Black
Clean your fountain pen nib, dip it and try that.

Once you have found the ratio you like, make up a reasonable batch.

Many Thanks
it seems obvious now but daunting to work out. So little black... i would have screwed it up with maybe a 50 50
Your advice is greatly appreciated

Yep - black really causes a dramatic shift in color. I would say 7 parts blue and 3 parts black would make a good strong blue-black.

You may get a nice color from your mixing, but this is probably not how Sheaffer makes their blue black ink. I don't know that much about Sheaffer inks, having only used their blue, but I do know that you cannot get Waterman Blue Black by mixing Florida Blue with Black.