One of my favorite beans is the Purple Podded Pole Bean. I haven't grown these for a few years because I wasn't able to find them, but this past spring I saw them in the Seed Savers catalog and had to have them.
Purple Podded Pole have very robust vines and are prolific producers. The flowers are a beautiful combination of lilac and purple, and the beans themselves are dark purple. (The flower you see in the picture on the left is not a bean flower - it's a morning glory flower that has closed up at the end of the day. Morning glories look very pretty growing with the beans.)
As you can see by the picture on the right, the beans can grow to about 9-10" long. At this length, a few of the beans have strings. However, they're not too long and really not noticeable when you're eating them. Picking the beans at about 7" long is probably ideal; however, they grow fast and one day can make all the difference.
Purple Podded Poles have a real old-fashioned home-grown bean taste. When cooked, the beans turn green. We normally eat our garden beans steamed with nothing on them, and they're great this way. You can also add a bit of butter, salt and pepper. Anything else seems sacrilegious!
These are prolific beans. At this time of year I'm generally picking about 1-2 pounds every 3-5 days. (My vines cover a trellis that's 6 feet tall and 5 feet wide.) This year I made the mistake of planting these beans under the snap peas. Because the snap pea vines were so thick, the beans didn't get enough sun to really grow well until I cut back the pea vines in early July. This meant the harvest time was probably 2-4 weeks shorter than it should have been. I won't make the same mistake next year!