What time in the spring should one prune blueberry plants?
Late winter to early spring.
Here is a link with more information.
https://www.gardeningknowhow.com/edible/fruits/blueberries/blueberry-plant-pruning.htm
I've read that I should prune out the 2/3 oldest branches but in the plant I have doing, that would remove half the plant! What should I do?!
For young plants you may only need to remove the fruit buds or the tips.
Here is a great link with more information.
http://www.fruit.cornell.edu/berry/production/pdfs/blueberries/bbprunerejuv.pdf
When should I put fertilizer on my blueberry bushes? I have had them for two years. I got them when they were three. I got zero berries last year.
You will want to fertilize your blueberries in the spring before their leaves have grown in with a high acid fertilizer. More information on blueberry fertilizer can be found in the following article:
https://www.gardeningknowhow.com/edible/fruits/blueberries/blueberry-fertilizer.htm
My blueberry bush had a lot of white flowers on it, but now they've all but a few disappeared. Upon closer examination, it looks as though they were picked off or dropped (although I don't see any flower on the ground around my bush). Could they have dropped from a brief cold snap we recently had? Or do birds eat the flowers, too? Also, could a fungal disease made the bush drop their flowers?
Depending on the stage of bud and flowering that the plant was in, the temperature drop may have affected your plant.
Here are some links that will help you.
http://www.canr.msu.edu/uploads/files/Bluerry_CycleOfGrowth_Mark.pdf
https://www.gardeningknowhow.com/edible/fruits/blueberries/growing-blueberry.htm
can you recommend an organic fertilizer that would work well with blueberry bushes and fulfill the needs that you have recommended?
This article give you both natural and chemical alternatives.
https://www.gardeningknowhow.com/edible/fruits/blueberries/blueberry-fertilizer.htm
Can I plant regular blueberries with Pink Popcorn blueberries?
Yes, you can plant them together, in fact, it's always a good idea to have at least two different types of Blueberries planted together.
https://www.gardeningknowhow.com/edible/fruits/blueberries/pink-blueberry-plants.htm
https://www.gardeningknowhow.com/edible/fruits/blueberries/growing-blueberry.htm
I have a blueberry tree but the berries that grow are very sour and have seeds in them. Is there a reason for this? How can I fix it? Thanks.
Sour blueberries are SUCH a disappointment! Hopefully you didn't (accidentally?) purchase a bush which purposefully produces tart or sour fruit. This article may help you: https://www.gardeningknowhow.com/edible/fruits/blueberries/sweeten-sour-blueberries.htm