Potatoes going black in the microwave

Can anyone tell me why, when I cook jacket potatoes in the microwave, they often go black inside? Sometimes its just in a small area, sometimes the whole inside goes black.
I do poke the potatoes heaps, and microwave until just cooked, not that the cooking time seems to make any difference. Sometimes they turn black before they are even cooked right through.
Is it the type of potato I'm using?

Well I never. I have learnt something today. I can't say I have had potatoes go black on cooking, but curiousity had me searching online to learn it appears it is most likely either from having been stored a too cold a temperature [even prior to digging up], or from bruising:

"Why do my potatoes sometimes turn black after I cook them?
This is one of the hardest things to detect when potatoes are brought in from the field and sorted to go into bags for consumers. Black spots just below the skin of the potato can occur if the potato is stored too cold (below 40 degrees) or when a potato is dropped more than 6 inches or something heavy is placed on top of them. The damage does not appear immediately but can become noticeable after one or two days in storage. Since the skin is not broken it is very hard to detect black spots until the potato is cooked.

Despite their hardy appearance, potatoes can bruise as easily as a banana or apple. While it might not be convenient, you can still eat the potato as long as you trim away the black spots before cooking. "

"We do not recommend storing your potatoes in the refrigerator, but if you do, letting the potato warm gradually to room temperature before cooking can reduce the discoloration. "

and I had never heard this 'theory' below -

" Does refrigerating white potatoes turn the starch into sugar?

A refrigerator's cool temperatures do encourage the starch in potatoes to become sugar. While this process makes potatoes darken and brown more easily during cooking, it doesn't have a big impact on the vegetable's nutritional value. It's a good idea to store fresh potatoes away from sunlight in a cool (about 45°F to 50°F), dry spot. "

So, thank you for asking your question!

Thanks.
I can only assume then, that the last couple of bags I have bought got frozen in the ground. I store mine in a cupboard so I doubt they get unduly cold in there. I have pretty much given up microwaving potatoes lately due to them turning black.
It does seem odd though, as I've been cooking jacket potatoes in the microwave for years but have only noticed them turning black in recent months.