A blue moon has a couple of definitions. One is when dust makes a full moon look blue to the observer from the ground, literally, because the dust collected in the atmosphere creates the illusion.
The second, more common, is when there are two full moons in one month. And, that's what we are expecting July 31, 2015 – a second full moon called a Blue Moon that doesn't actually look blue.
(Yes, there is a third -- a Blue Moon is a popular beer, but, we assume here that people are searching for lunar answers, not ones relating to hops and craft beer. If you want that, go here...)
From NASA: "According to modern folklore, a Blue Moon is the second full moon in a calendar month. Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens every two and a half years, on average."
"There are two definitions of a Blue Moon in astronomy; both are a type of full moon. If the moon actually looks blue, it's caused by a rare
type of dust in the atmosphere," according to timeanddate.com
Since a full moon occurs every 29.5 days, it isn't a normal occurrence that two full moons appear in the same month. But July 2015 is one of those rare months, with two full moons – and thus, July 31 2015, the second full moon of the month, will be a Blue Moon.
From moongiant.com: "Depending on the exact time of the Blue Moon it is possible that some places in the world don't technicly have a Blue Moon. As an example the Blue Moon on August 31, 2012 occured exactly at 13:58 UT. The Blue Moon will occur on August 31 for South America, North America, Europe, Africa, Asia, India, and Austrialia but New Zealand will just miss out. For New Zealand the Full Moon occurs just past mid-night on September 1st."
"Astronomical Blue Moons happen either once every two to three years or so, depending on which of the two definitions you apply.
A Moon that actually looks blue, however, is a very rare sight. The Moon, full or any other phase, can appear blue when the atmosphere is filled with dust or smoke particles of a certain size; slightly wider than 0.7 micron, for instance after a dust storm or a volcanic eruption," according to timeanddate.com.