Blueberries.

Growing and making jam with blueberries | Life and style | The Guardian
The only way I'll ever have enough blueberries for preserving is to grow my own.
We don't seem to get them in any quantity in the UK and those cartons in the supermarket are far too small and too expensive to use as a serious jam ingredient.
Sometimes I've been lucky enough to come across berries with a yellow sticker at a bargain price that are close to their 'sell by' date but still fresh enough to make use of. I buy them up and fill the freezer.
When preserved in combination with another fruit, a little goes a long way, so is a good compromise, but I'd love to have a plentiful supply to feature blueberries as the main player.
For that reason it's time to plant some blueberry bushes on the allotment.

Planting for preserving has specific considerations.
You are aiming to create a glut rather than avoid one and instead of varieties with staggered ripening times, you want enough to harvest all in one go.

Most blueberries you buy are also fairly sweet and nondescript and for jam making a tarter fruit works better for a distinctive flavour once cooked.
As blueberries benefit from company, it is advised to grow at least two varieties side by side for higher yields.
I've chosen Darrow and Jersey, both attributed with a sharper flavour ideal for cooking and I already have Bluecrop growing neglectfully in a pot, that deserves to be liberated.
As they need acid soil to prosper and my soil isn't quite acid enough, I've dug out each planting hole and filled with a mixture of ericaceous compost mixed with about a quarter bark chippings.

I'll be mulching them too with coffee grounds and composted pine needles and watering them only with collected rain water if need be, to keep them happy.

In the meantime, here is one of my favourite recipes using some shop bought berries alongside seasonal rhubarb.

Makes 1.25kg
600g rhubarb
300g blueberries
2 limes, the zest and juice
700g sugar

Wash rhubarb, top and tail then chop into evenly sized pieces (I usually run a knife down the middle of the stalks then chop into roughly 1cm sized pieces).
Place in a glass bowl and pour the sugar over the top.
Cover with a plate or cling film and leave overnight, by which time the sugar will have soaked up the juice from the rhubarb.

Place the grated lime zest and blueberries in a pan, adding 3 tblsp of lime juice.
Heat gently and simmer for 15 minutes with the lid on, until the berries are cooked and surrounded by juice.

Pour the rhubarb and sugar into a jam pan and stir over a low heat until the sugar is completely dissolved.
Add the blueberries and lime, turn up the heat and cook at a rolling boil until setting point is reached (a small dollop of the syrup on a cold plate will readily form a skin when left to cool slightly).
It took me 10 minutes to achieve this and I advise that you keep an eye on it and give the occasional stir whilst it cooks as the mixture is apt to burn if you're not careful.

Skim if necessary.
Pour into hot sterilised jars, put a circle of waxed paper on the surface of each one and seal.

This jam has a nice soft set that suits me fine.
If you prefer your jam to 'cut' rather than dollop substitute all or half of the sugar with preserving sugar that includes added pectin.