Would it be unthinkable for a medieval European-esque noble — say, the son of a baron — to take up blacksmithing as a hobby? How much might their family and friends disapprove, if at all?
Maybe to piss off his well-to-do parents or peers who expect him to partake in more intellectual or political pursuits. Possibly also to show up his father as a weakling?
I would guess his family might not care much as long as he keeps it to himself, but for a nobleman to be seen doing a commoner's job would make the whole family look bad. If other nobles got hold of that, they would certainly think ill of his father for allowing him to do it.
That was my initial suspicion as well. The thing is though, there's an artistry to metal working, and it strikes me as odd to keep an artistic outlet out of the hands of the nobility. Would the baron not appreciate a beautiful dagger forged by his son, or would he only feel embarrassment?
Probably depends on the baron and his dedication to social propriety. I'd think it might be more of an issue for 19th century aristocrats than medieval feudal lords, but who knows?
I'd actually think the other way. In medieval times intermingling with the commoners was definetely frowned upon, I think more than later in history. In recent centuries nobility in Europe relaxed quite a bit, and most noblemen were encouraged to explore artistic and intellectual endeavours, and just generally to find hobbies to fill their spare time. Blacksmithing would probably be seen as a quirky thing to do for a young noble in the 19th century.
First of all it's a question of the era we're talking about, the medieval era spans a lot of time that by no means was static. So depending on when it's entirely possible for a lord to behave in such a way without it being too much of an anomaly. But a few points: Nobles did not necessarily have a lot of free time. Depending on the era they've still got military and administrative responsibilities, and learning blacksmithing would be an additional draw upon their time. Depending on whether or not there are craftsmen guilds around, it's likely that getting actually educated in blacksmithing would be rather tricky. And given that most trades were handed down from master to apprentice, finding out the "secrets" of smithing could be another hurdle. Metal is also a valuable resource, so it would have to be a wealthy or frivolous lord who'd be willing to spend the time to make sup par products. On top of that, medieval blacksmithing is a rather unhealthy, it's incredibly rare to get pure iron and combined with poor ventilation there were a lot of blacksmiths who were considered "touched" due to chemical poisoning adversely affecting their mental health. Blacksmithing is also a rather catchall term, it covers farriers (horseshoe makers), armorers, sword-smiths and blacksmiths (who would make cauldrons, latches, pokers and the like, or plows and farmers tools). It's entirely possible that a blacksmith of some variety could have been given a minor lordship and kept up with their skills to a certain degree, but as has been pointed out for a lord to learn it would be a bit of a social gaffe. Lords have a duty to govern and lead, the serfs/freemen have a duty to toil, and for a lord to behave like their lessors would reflect incredibly poorly on them. If however, we're talking about the child of a lord, then there's a bit more leeway depending on how far removed from the line of succession the child is. It was generally desirable to have at least a spare son in case your heir met with an unfortunate accident, but once you get to third sons and younger, it tended to be that they'd get shipped off to the church, as once they'd taken their vows they'd be out of the inheritance game and thus less likely to cause any more trouble. One of these younger children would have a much easier time sneaking off to learn to become a blacksmith. But it would probably earn them the status of black sheep of the family fairly quickly.
I feel like I must mention here that old Prince Nikolay Bolkonsky in War and Peace is one of the more beloved/iconic characters and he's a hobby wood carver with a dedicated room in his palace. Mind, this pertains late 18th / early 19th century at which point nobility was so ridiculously wealthy and powerful they could afford virtually anything.
Tradesmen were very respected in the middle ages. You did not turn your nose up at a tradesman. So they probably wouldn't disapprove of the activity of blacksmithing itself, per se. What they would disapprove is the negligence of other, more important endeavors such as swordsmanship, or learning to be a proper steward.