What to look for when buying good quality loose black tea?

Every tea can have potentially various grades and different teas will have different qualities. I'm not sure what is available where you are, but if you're fairly new to drinking loose leaf teas, I'd for starters find a store that lets you see, smell and possibly sample various teas. Once you have a better idea of what you're looking for, then online dealers will be easier to navigate.

1. So, firstly, find a reputable store, one where they are knowledgeable about their product, and difference in grades.
2. Ask to smell the teas. Please DO NOT stick your head in the tea, waft it with your hands or the lid of the tin. If you like the aroma, it's a good starting indicator of how you'll like the tea.
3. Look closely, are the tea leaves fairly even in size? An unbroken leaf is generally graded higher than a leaf that is broken/crushed. Though since you're specifically asking about black teas, note that certain black teas are deliberately cut, to encourage a stronger flavour. If the leaves are torn, crushed, it may be a sign of a rough machine harvest, or poor handling. This could result in bitterness. The lower grades (small crumbs and dust) generally end up in tea bags. So, note that.
4. Most importantly, taste it! If the store offers a few different sampler sizes, go for that. Ask the people there, about how they'd recommend you prepare it. And try a few different kinds and grades.

Note, if you're new to loose leaf tea, it should taste different than what you're used to. Trust your taste buds and your nose first... Regardless of what it looks like and the price and what the description says, if you don't like the way it tastes & smells (which is often subjective) the rest is pretty meaningless.

To back up what I say, I can site lots of articles that said said all sorts of things. Some that made sense, others not. Although I put it all together as such. Some black teas are premium ones (labelled Assam, Ceylon, Darjeeling, Nilgiri, Keemun, Souchong, and Yunnan). Also estate Africa blacks and good grade South/Central American ones which they also have today are good as these are generally orthodox teas. Thiland, Vietnam and Burma also produce higher quality ones. They are called flowery, tippy, golden or othodox. Most places in the world use these with the exception of North America, UK, Belgium, Egypt and some although not all or most middle Eastern countries which use CTCs a lot. Countries using orthodox though tea articles never say it directly although indicate to it if you read lots of other articles to get it -are always satisfied with their blacks.

Blended teas are not always lower quality necessarilty than single leaf estate ones. Common sense says that if you mix decent quality tea you get a decent blend. If the blend contains cheap teas you get a cheaper blend. This is common for general supermarket teas. French Breakfast tea for instance is often made of a higher quality blend. Whereas you can get good English Breakfast blends - most of them use cheaper CTC teas for the bag. This is the case today with British breakfast blends (English, Irish and Scottish) since convenience became more of an issue here. Also Earl Grey is often used in cheaper blends though one can buy expensive ones. Masala Chai blend is probably the cheapest as this is often what the dust grades are used for though they can taste quite good in it. Orange pekoe can follow suit with that although it is possible to have very good ones (flowery grade ones) which are far better than masala and most chai blends are sometimes cheapest even though they put the word specialty on it on a cheap tea set. Most in the supermarkets however are the dust grades. Orange pekoe can also refer to a leaf grade size for certain types of (often Ceylon teas) although at full is a nice flowery-bud tea. Also keep in mind that green and many herbal teas in bags are also usually these days very low quality and bland and some even below the former though I have seen it according to my observations outside

Flowery, (F) Tippy (T) and golden (G) are high end blacks. B means a broken leaf which still can be a good tea, especially if listed after the previous notations or the OP sign. If an F is listed after the B however its likely a less good tea as that means a fannings which is a lower grade. D or dust is the lowest. Flowery golden tippy are orthodox. Broken can be if not a fannings and dust. The latter two are CTC grades. They can however work well IF you want to make masala chai or some sweet iced tea, just for the sake of it. The higher end blacks are loose leaf and have more of a malty brisk flavor yet they take a couple of minutes to brew up. CTC teas brew up faster in the bag which is what people like although they never as good as an estate orthodox one