Pulmonary Embolism Support Group
By far the most common form of pulmonary embolism is a thromboembolism, which occurs when a blood clot, generally a venous thrombus, becomes dislodged from its site of formation and embolizes to the arterial blood supply of one of the lungs. Symptoms may include difficulty breathing, pain during breathing, and more rarely circulatory instability and death.
The articles that talk about blood in the stool (red---- not much to worry about and usually a hemorrhoid causes it--- black---- it's usually described as black and tarry and is 'old blood' and a matter for concern.)
You used the words 'little' and 'speck' so I'm going to give a guess that it might be some food passed through undigested. Coarse ground black pepper? Black sesame seeds? Bits of spinach, which can look black? Other foods or spices?
Having said that----- remember those are just my guesses.
If you can stand more grossness and if you're really worried---- try to save the 'specimen' to take in to the ER.