no explanation about the blue though......a wild guess.....to block some of the light w/o excessive heating as a black bag would do so a compromise.
https://rccostarica.wordpress.com/2009/12/16/going-bananas/
Another link says this: "Tip: commercial banana growers use bunch covers (plastic bags open at both ends that they slip over the bunch and tie at the top) to protect bananas from diseases, insects, sunburn and marauders. You can try to buy those bags at a rural supplies store, or beg some of a grower"..........didn't think about them being open at the bottom..................
Growing Bananas - How To Grow Banana Plants And Keep Them Happy
...and another......"Bagging your bunches serves a number of purposes. It holds in the available heat which is important during cold or cool weather. Bananas also give off ethylene oxide as they ripen. Bagging keeps that gas close to the fruit, which in turn accelerates ripening. It prevents fruits from developing blemishes. The plastic also serves as a barrier to keep out tree rats and other pest species that like to feed on bananas."
and still another...........
How to Protect Bunches of Banana Fruit | Home Guides | SF Gate
"It takes an additional four to eight months for the fruits to ripen. Flowering and fruiting times depend upon variety. This is a long time to wait for delicious banana fruits, and it would be a shame to lose them to birds, insects or elemental damage. Even the plant’s own leaves can blemish or damage the peels, reducing the fruit’s resistance to rot. Commercial producers use expensive plastic banana bunch bags that often are impregnated with insecticides to protect banana bunches. You can protect your bananas with cheap plastic garbage bags.
"Cut the bottom seam off a large plastic garbage bag with scissors. Choose a clear bag if the bananas are protected from sunlight by the plant’s leaves. If the fruits are exposed to strong direct sunlight, use an opaque white garbage bag.
4
Slide the plastic garbage bag up over all the banana bunches. Gather the end of the bag around the stalk just above the top bunch of fruit. Secure it to the stalk with a piece of string. Leave the bottom of the bag open to allow for air circulation. This also keeps the fruits from overheating inside the plastic covering."