I can add that if you still insist on blue, for whatever reason, that you
might want to think about expectations consumers might have about the drink.
The preference of the flavour might turn out to be higher if it matches the
expectation. Many times bartenders create drinks without thinking about these
expectations and don't create drinks guided by consumers wishes but instead on
random trial and error accompanied by artistic intuition. Artistic intuition
may be nice but it is very personal and subjective and my not always result
into something which complies with consumer demand. Are you making this drink
for your own pleasure or for pleasing consumers and increasing profit?
My experience is that blue colour matches the less sweet drinks or at least
drinks with some good dose of sourness or astringency (Producers of blue
sports drinks have it wrong. The red ones and the orange match the taste much
better. And they are better.). It may be associated with exploration and also
with risk. People who choose to drink something blue might tend to prefer
strange and new flavours (hence the fantasy flavour in blue sports drinks).
There would be disappointment if your blue drink points out to be a drink
which is little dynamic and has simple flavour.
I would personally use blue curacao to create a blue drink. Hypnotic is
another popular blue ingredient/drink. But, besides low availability in my
area it tends to be less interesting for mixed drinks (It is a good ingredient
to create drinks but it is already a lot polished and is, as such, not a good
building stone for ones own creations. Especially when one has restraints like
the need to use pineapple and gin as well). Other blue ingredients are rare
and pure colourants are not my (unfounded) personal preference.
A problem with pineapple might be that it is very turbid and has a strong
yellow colour. Combinations of blue curacao and pineapple might turn green.
You may want to think about a layered drink or use low amounts of pineapple.
Another reason not to use a blue drink for a drink competition (besides the
ones put forward by Merlin): Competitions are decided by the opinions of only
a few judges. Their taste preferences are personal and subjective but also,
highly influenced by circumstances. One of these circumstances are
preconceptions about how drinks should look like. If a competition drink
deviates too much from what people commonly believe is good drink then they
will rate it as less high. A blue colour is not a common colour (especially
for drinks believed to be fashionable and haute cuisine) so there is a
reasonable chance that the judges will rate it a less high because of the less
common colour.
For inspiration, look at those websites which have a feature to search in a
drink database by selecting ingredients. The bolscocktails.com database lists,
for instance, 20 recipes with, bc, pine jc and gin among which the blue canary
(1 part bc, 2 parts gin, 3 parts pine jc). This website,
http://www.webtender.com/cgi-bin/inmybar , boast a very large list of drinks
(select the three ingredients and use the option 'can miss three'). Keep in
mind that these databases are often such large that they are almost as close
as containing every possible drink to be made. They are not always a proper
standard for good (beyond nice) drinks.