I am so sorry fir the sudden loss of your puppy! Just death and the lack of circulation could be the cause. However, it could have been lack of oxygen from choking or possibly heart problems. Yes, could be poisoning so do check your yard thoroughly.
Unfortunately, there is no way to tell unless a necropsy is done. If you suspect poisoning, and I hope that is not the case, I would check with neighbors to see if anyone has had a problem or put out poison rats, etc
Sometimes,rodents ingest the poison but can live long enough to wander off and if an animal ingests the poisoned rodent, they get poisoned.
Hope you can get some answers. Again, so sorry.
Please do come back and update.
I really doubt that he would die instantly from poison. Generally an animal will show signs of an illness if poisoned, although in some cases the illness can be very brief. I think you would have known about a bite, also. Some breeds of dogs are prone to heart problems that cause sudden death in the very circumstances that you describe, though. The tongue turning black would have been from lack of oxygen when the heart stopped. I have some friends who had a one year-old puppy who they were playing ball with. The owner threw the ball for the dog, and the dog ran out to get it, and then he turned around and looked at the owners for a few seconds and just dropped dead. He was fine up until that moment. It turned out the dog had been born with a heart defect that never caused any symptoms until it killed him. The only thing that I can think of for you to do is, if it was a purebred dog, to try to find out if this is a known health problem in the breed. In any case, it's very sad, and I'm so sorry for your loss.
Thank you Margot49 and Skydnsr for your advice. I did search my yard completely. I also look up my puppy breed (Mini-iltian grey-hound and jack russell) which I'm the owner of both. Yes, I believe he did have a heart problem. I'm to have his body check by a vet. Since it's only one left of my jack russell litter, I'm going to take the one I gave my mom check. The hardest part was telling my God-Baby who artisic her puppy past...we both cried. Spike was meant for her and she was really working hard to focus with everything she dose in life. Thank you again for both comments. Have an awesome day!
Good idea to have the other puppy checked. I hope you will be able to find peace. If, by chance, you have never heard the story about the Rainbow Bridge, here is a link to it. http://petloss.com/rainbowbridge.htm