Okay, so total newbie been in the hobby for about 11 months, still learning and reading about it.

can anyone suggest a suitable blue for painting British Napoleonic artillery uniforms in 6mm of course

Assuming you mean the guns and trails I painted mine Tamiya Neutral Grey XF53 followed by a shading wash.

ooop's apologies should have said the uniforms

Then Tamiya flat blue XF-8 with a shading wash. I undercoat my figures white as I've found if I use black I can't see any of the detail to paint.

On my side, I always paint minis with Prince August/Vallejo acrylic paints. I never succeed using Tamiya/Gunze paints with a brush (even on standard models - for me they are wonderful with an aerograph, but cloggy(?) for brush painting). Main color I use for british artillery is (with PA = Prince August ref, VJ = Vallejo ref) 
Prussian blue (?) : PA 965 / VJ 51
For guns support(?) I use "turquoise clair" (?) PA 840 / VJ 68
Questions marks are because my paints (such as I ;-) ) are labelled in french, so not sure about the exact translation.
Hope this helps.
Here is a quick picture I took for you to make your mind.
As taken with a mobile phone colors are not that top (uniforms should be a bit darker) but tonalities are somewhat acceptables.

thanks