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Hopefully, these 2 links will answer your questions:
http://aptgarden.blogspot.com/2011/07/how-to-harvest-bok-choy-seeds.html
http://forums.gardenweb.com/forums/load/asianveg/msg1000104029643.html

It is caused by a phosphorous deficiency. Some bone meal will help correct it.

This article will help with growing bok choy: https://www.gardeningknowhow.com/edible/vegetables/bok-choy/planting-bok-choy.htm

If your cabbage and bok choy have flowers, then they have bolted to seed and are passed their prime. They like to grow in cool weather. Too hot and they will get bitter and bolt (start producing flowers and seed). While edible, you can try to eat them, but they will probably be tougher and more bitter. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/vgen/what-is-bolting-what-it-means-when-a-plant-bolts.htm, https://www.gardeningknowhow.com/edible/vegetables/bok-choy/preventing-bok-choy-bolt.htm, https://www.gardeningknowhow.com/edible/vegetables/cabbage/cabbage-head-formation.htm

When it goes to seed, this is called bolting. After bok choy bolts, it is considered to be done and the whole plant is harvested, regardless of the size. After it bolts, it will cease to send energy to the leaves, even if you cut the flowers off.

You can help prevent bolting in the future by keeping the roots as cool as possible. Heavy mulching and frequent watering will help with this, but growing it in the cool seasons (spring and fall) will help the best.

This article describes many diseases of bok choy, and the treatments for them: http://edis.ifas.ufl.edu/pi107