Black tea is less delicate than green tea. For instance, black tea a couple years old would be perfectly fine if it had been stored properly.
However, the black tea of today is more delicate than the black tea of yesteryear. This is not necessarily a bad thing; the prolonged drying process of very shelf-stable black tea tends to remove some of the more delicate flavors, and also tends to give the tea an odd flavor if not allowed to rest for a few months.
As a general rule of thumb, tea will be at its best if it is drank the same year it is grown. This is why it is wise to buy tea from merchants who have this information available, and are upfront about it. The biggest enemy in tea storage is moisture. Once dry tea reaches a certain moisture content, it becomes vulnerable to many other problems.
Tea quality is determined by a mixture of objective and subjective measures. For instance, British tea standards of quality are very different than Chinese standards, because the way the tea is prepared and enjoyed is quite different.
On the objective level, the basic indicator of quality agreed on by most is the plucking standard. When a tea uses mostly young leaves ("Two leaves and a bud") it is considered to have a better flavor, than when it uses more mature tea plants ("Three leaves", "Four leaves", etc.) Unfortunately, this information can be tricky to coax out of the tea leaves themselves, unless you are drinking certain types of whole leaf tea.
Some basic ways of determining black tea quality:
Look at the dried leaves -- do they have a liberal amount of yellow tips? This is a good sign, but not always. Some manufacturers will take a poor quality tea and spike it with tea tips. Also, a tea that is all tips will have a fairly bland flavor, although it will be high in caffeine; it is best to simply look to see if there are tips there.
Is it free from loose stems and twigs? Stems and twigs do not have the same flavor as tea leaves, and a tea high in stems will not taste right. Note, some whole leave tea makers will roll the entire two leaves and a bud by hand with the stem -- this is not so bad, as it shows the plucking standard in the steeped leaves. It is loose stems without leaves that are a problem.
Is the tea an even grade? That means, if it is whole leaf, are all the leaves whole? If it is broken, are the pieces all the same size? If tea fannings, are the fannings the same size? Is the tea "dusty"? Tea that is a mixture of sizes will steep unevenly, the small parts steeping too fast, and the large ones too slow.
Some black tea may have a fine fuzz on it -- this is not dust, but pubescence of the leaves, and is usually a good sign. (Just make sure it isn't mold.
Related to this, check for mold. Mold should only be on Pu-erh, not black tea.
The tea should be dry -- it should break cleanly rather than bend when you take a few dry leaves. If it has gotten wet, it will likely taste odd.
And of course, if you can sample the tea, that would be best. The proof is in the flavor.
Once you have tasted the tea, don't neglect looking at the steeped leaves; they can tell you additional things about the tea.
Note, these are all only to demonstrate quality that is already there -- some areas that grow tea struggle simply because their land does not produce tea with potential, no matter how carefully they prepare the green leaf. This is why it is good to ask where a tea is from. Generally, young tea leaves from mature bushes, grown on good land in high altitudes, have the most potential to turn into good tea. However, tea is an agricultural product, and sometimes the difference between a good tea and a bad tea will come down to accidents of the season and accidents of nature.