How to design an experiment to measure coffee preferences?
The study design sounds pretty good. Some of the good things you are proposing:


Using a repeated measures design will give you more statistical power than a between subjects design, which is particularly useful when your sample size is small. 
Randomising or counterbalancing for order should mostly control for order effects.
Double blind will focus the test on taste rather than branding and as you say limit experimenter effects


A few further suggestions:


You could provide a little bread and water between the two tastings or separate the tastings by some longer period of time to limit any residual taste effects from the first taste.
And yes, as you say, increasing sample size will increase your statistical power. That said, if your sample is your population (e.g., you have an office of 10 people), then the smaller sample size doesn't matter.
You should consider how the response scale you will use for rating taste. You could simple get a binary preference. But I think it might be better to use some form of scale that rates preference in addition to an explicit question at the end about preference.