Is this blight?

[Maggiethecat]

The leaves and stem of this plant seem fine but not sure if this isolated black spot is blight?

No.

That's blossom end rot.

It happens quite commonly in tomatoes. Technically, it's a deficiency in calcium, but as calcium is a pain to move around the plant, the tips of the plant (far edge of the tomato) often don't get enough.

It's mainly either because there is not enough calcium, but USUALLY because airflow isn't high enough. Air movement = greater transpiration (movement of nutrients around the plant), so in greenhouses that are hot without vents open etc, you'll see this.

It's blossom end rot, caused by low calcium levels usually due to irregular watering.

Phew!! thank you.
It’s a Moneymaker that I’ve not managed to plant in a proper pot so is probably underwatered in its small pot. Will repot this weekend and perhaps give it a feed.

As a rule, the bigger the tomato, the more likely this is to happen. Water/calcium have a long way to go through the plant to reach the apex of the tomato fruit, so the bigger the tomato...

Do you have an automated dripper? It can slightly be mitigated by spraying the leaves with a calcium supplement, but if you're doing that regularly, you might as well water it while you're there! - unfortunately by the time you see it, you've already passed the 'could have done with some calcium' stage anyway!

You can make a drip irrigator from a plastic bottle if you don't want to install an automatic watering system.

https://www.wikihow.com/Make-a-Drip-Irrigator-from-a-Plastic-Bottle