No, it is extremely uncommon and highly abnormal behavior. Such a situation goes against all norms of healthy family relationships and basic moral and ethical values.
Here is a story. The son finds out that his mom has been hiding some family heirlooms that were supposed to be passed down to him and his siblings. He then tries to blackmail her by threatening to expose some old family secrets if she doesn't give him the heirlooms immediately. The mom is shocked at first but then sits down with the son to explain the real reason behind hiding the heirlooms, which was to protect them from being sold by the son's irresponsible uncle. Through this conversation, the son realizes his mistake and the blackmail attempt stops.
One possible story could be that the son found out some secrets about his mom and aunt, like a financial misstep in the past. He then tries to blackmail them by threatening to tell the rest of the family. But this is a very unethical behavior.
There could be several reasons. One possibility is that they are in some kind of trouble themselves and think blackmailing the mom can solve their problems. For example, they might have debts they need to pay off. Another reason could be that they are just bad - natured kids. They might enjoy having power over someone and see blackmail as a way to get that power. Or perhaps they have some secrets that they don't want the son to know and they are using blackmail to keep the mom quiet about those secrets.
One reason could be financial gain. Maybe the son wants more money for luxury items or to pay off debts and he thinks blackmailing his mom is an easy way to get it. For example, if he knows she has a secret savings account she doesn't want anyone to know about.
If the mom is reluctant but still doing the blackmail, it's likely that she's in some sort of difficult situation herself. Maybe she's in debt or has some secrets she doesn't want exposed. However, blackmailing her son's friend is completely wrong. The friend might choose to confront the mom and try to reason with her, or he could cut off all ties with the family which would be sad for the son.
Families can prevent this by promoting open communication. If everyone in the family feels comfortable sharing things, there will be fewer secrets for a son to use for blackmail.
No, it's not a common theme. In most stories, the relationship between a mom and son is based on love and respect. Blackmail is an extreme and unethical behavior that goes against the normal family values, so it's not something that is frequently explored in typical hot stories.