Disclaimer: I describe only the things i have experience with, or with things similar enough for me to be sure. There are many ways I heard but have no idea about, so feel free, encouraged even, to post your own answers.

Specifically for black currant, there are things you can do with it, and expected results:

Adding during fermentation

Juice or must (squeezed fruit)

It will add flavor and aroma, but might also introduce wild yeast and bacteria. Probably will result in additional sourness, and maybe "horsey" or "rustic" smell. Some like it, some don't.

Sugar content similar to apple juice, should not affect ABV significantly.

Juice or must pre-heated / boiled

It will kill most wild stuff if done properly, but will also purge volatile aromatic compounds, leaving only the strongest ones.

Due to evaporation, sugar content might be raised, and this will affect final ABV, but probably only slightly.

Frozen fruit

If you can get it, it probably was sterilized in factory. Wash thoroughly to be sure to remove sterilizing agents.

Most of flavor should be kept, and sugar content should be intact.

Commercial black currant syrup

Stay away from it. Between 0.5% and 2% real black currant, usually, and 60% of fermentable sugars, it is a good way to raise ABV horrifically and ruin taste. That is, if artificial flavorings and preservatives will not kill your yeast first.

Homemade syrup

If you use polish traditional recipe, it might be not that bad:

• Boil just enough water to comfortably cover your fruit
• Add sugar, 1kg sugar for 1kg fruit
• Boil until sugar is well dissolved
• Turn off heating
• Put fruit in hot sugar solution
• After 5 minuter, pour it thorough a sieve (or not). Cool it by putting pot in cold water.

This syrup will raise ABV significantly, with good 30% to 50% of sugar in it, but it has pretty unlimited shelf life and sugar helps to keep aromas in. Might give you decent results.

Organic extracts

Powder ones are basically a sterilized, freeze dried juice. Easiest, surest, fastest, most expensive way. As for the taste - think the difference between espresso and instant coffee. Given it's water free, you can add as much as you want. Just check the sugar content - some varieties are sugar free, and will not affect ABV in any way. Some have part of fruit sugars in them, and you need to remember that in your calculations.

For fluid extracts - I have no idea, but probably it's quite similar.

Dried fruit, fruit tea and similar

From my experience - NOOOOO!!!. The only time I poured whole batch down the toilet was when I tried to use dried herbs and fruit. I thought I did enough to sterilize it, but apparently I didn't, and I got infection going, big time. Pretty nasty infection, worse than you get from natural yeast and bacteria on fresh fruit. Your mileage may vary, of course. But don't try asking me for help if you will try that, I can't do it right.

On the other hand, Red Tiger in comments reported to get this done with success. He uses disinfectant tabs or makes tinctures, and also waits for beer to be fully fermented to avoid feeding bacteria that might survive. For him, it works. For me, it didn't. Your choice.

Adding after fermentation?

As Mr. Boy pointed out, if you will add these after fermentation, with yeast safely killed, you will retain more sweetness, more flavors, and will not affect ABV. On the other hand, if yeast are still alive, you might end up with exploding bottles. And of course forget re-fermenting in bottles, impossible without yeast. Sugar-free extracts are exception, of course, and can be added with live yeast in cider safely (if they are honestly sugar free).

If you want only slight currant flavor, you may use juice as priming sugar. It will hardly be strong enough to sniff it, but it will add to the "richness" of your brew if used that way.