We searched around on this- it wasn’t obvious and we remain uncertain. But we can provide some information now while still soliciting advice from people in both the US and UK.
1) It could be a virus (UPDATE BELOW)
2) It could be nutrients
1) In one of your pictures ("Noticing this discoloration as well") the veins are clearly visible. This sometimes indicates a viral infection. These would be vectored by aphids or some other sucking insect or possibly from the nursery you got them from.
The symptoms are similar to Tomato Ringspot Virus which does occur in currants http://www.ars-grin.gov/cor/ribes/rib...
See some details here on this virus https://www.eppo.int/QUARANTINE/virus...
It seems more likely that your plants are lacking certain nutrients. Currants (Ribes) have very shallow roots and if it is well drained soil they can be nutrient poor very quickly.
This is a bit on what soil they tolerate
"Currants and gooseberries are fairly tolerant of a wide range of soil conditions and less-than-perfect sites. They perform best in well-drained silt to sandy loam soils with organic matter content greater than 1 percent and good water-holding capacity. Planting in light sandy or heavy clay soils should be avoided, as well as areas in which water stands for any length of time. If the site is poorly drained, improve it by tiling or building raised beds. When using raised beds, monitor soil moisture as they can dry more quickly than level soil areas. Both heavy and light soils can be improved with the addition of organic matter. The ideal soil pH is moderately acidic, from 5.5 to 6.5. Micronutrient deficiencies may occur at an alkaline pH greater than 7.0. Saline or salty soils near coastal areas should be avoided."
page 4 http://pubs.ext.vt.edu/438/438-107/43...
You should check your soil
“A soil test should be done to determine the soil pH and phosphorous and potassium levels and needs. These nutrients should be amended to moderate levels, with available phosphorus brought to a range of 50 to 75 pounds per acre and potassium to 150 to 200 pounds per acre. If pH levels are below 5.5, lime should be add- ed to bring the soil pH to 6.0 to 6.3. Along with lime, phosphorus can be incorporated in the fall. However, both potassium and nitrogen (25 to 35 pounds per acre) should be incorporated in the spring to avoid the loss of nutrients to leaching. Note that currants and gooseberries are sensitive to the chloride contained in muriate of potash (0-0-60) so another form of potassium, such as sulphate of potash, should be used. For smaller plantings, a general purpose fertilizer such as 10-10-10 or similar analysis can be used, and it should be broadcast and worked into the planting area, using approximately one pound of material per 100 square feet. Currants and gooseberries respond well to organic amendments, which in all soil types improves aeration and drain- age and increase water-holding capacity. Organic matter can be applied in the fall or spring before planting. Well- aged manure at four to five bushels per 100 square feet (1,750 to 2,200 bushels per acre) is a good option. Other suitable additions include finished compost, leaves, rotted hay or straw, shredded peat, or sawdust”
“When available, manure or other composted materials with significant nitrogen content (3 percent to 5 per- cent) are the best nutrient sources for ribes, which re- spond well to slowly released organic nitrogen sources.”
So, what nutrients might you be missing? Some nutrients, such as potassium or magnesium, are highly mobile within a plant. If need be, they are quickly transported to younger parts of a plant, resulting in deficiency symptoms mainly on older leaves.
In Currants magnesium and potassium deficiency look similar. Chlorosis between veins (like the picture you showed). You can treat magnesium with dolomitic lime, which is a mixture of magnesium and calcium carbonate. Too much reduces the absorption of potassium, so be sparing at first.
Here is a great resource on what the different nutrient deficiencies look like. http://5e.plantphys.net/article.php?i...
I would suggest applying fertilizers/supplements and seeing how you get on.
It would be useful to record your progress in our plant journal (see under your profile) so we can see how you get on
UPDATE MAY 26TH
I contatced Dr Joseph Postman | Plant Pathologist - Curator
NATIONAL CLONAL GERMPLASM REPOSITORY
USDA Agricultural Research Service
33447 Peoria Road, Corvallis, Oregon USA
"The single leaf with yellow veins looks like typical symptoms of Gooseberry veinbanding virus, although aphid feeding can also induce symptoms like this, so check for presence of aphids. The general yellowing of leaves can have multiple causes. One possibility for black currants is a disorder called “Black currant yellows”. No virus or other pathogen has been associated with this disorder. It may be genetic like June Yellows in strawberry, or it may be caused by a virus that has not yet been identified, though this is unlikely as small fruit virologists have repeatedly examined affected plants. Certain black currant cultivars regularly exhibit the “yellows” symptoms in the spring, and then produce normal green growth as the season progresses. Do you know what cultivar is showing these symptons?"