Four minutes after the interval the hot-headed Jambos player was shown a straight red card by referee Stevie O'Reilly for what looked like a reckless challenge on Well midfielder Keith Lasley.
The Steelmen capitalised on their advantage on the hour mark when striker Jamie Murphy converted a pass from Nicky Law before Lasley, already booked, was sent off in the 86th minute after picking up his second yellow card for a foul on Hearts substitute Arvydas Novikovas.
Sergio hinted that he thought O'Reilly had made the wrong decision as he backed his player.
"I don't want to speak about the referees, we were advised that we shouldn't," said the Portuguese coach.
"But I think he was fighting for the ball. Maybe he touched the opponent but he is an honest player.
"I like Black's style but he is not just an aggressive player, he is a good player, he is intelligent and I am sure he will help Hearts in the future.
"I didn't think we should have lost but the second half was hard for us."
After watching his side leapfrog Celtic at the top of the Scottish Premier League with seven points from three games, Fir Park boss Stuart McCall also took a lenient view of Black's misdemeanour.
"I have just seen it again (on television) and I think both players are committed and they both went in for the ball," he said.
"I don't think there is any malice at all from Black, I think Keith will say the same.
"But he (Black) has got there a split second late and it can look reckless and probably the referee sent him off for a reckless challenge.
"If he had got there a split second later it would have been a good 50-50 but in these conditions if you go to ground in a sliding challenge you have to get the ball."
However, McCall was frustrated by Lasley's dismissal near the end.
He said: "Lasley was really disappointed about the second one (yellow).
"At the end of the day, fouls are not always yellow cards.
"It is a free-kick. I can understand why the referee sent him off but sometimes we have to realise that every free kick isn't a yellow card."
The former Rangers and Scotland midfielder played down his side's lofty position, adding: "It is a good start. It is obviously what we are looking for but it's nothing more than that.
"We have to keep building on it and we look to the next game against St Mirren and nothing more.
"Hearts had the better chances in the first half which came from us giving the ball away sloppily.
"Obviously the sending off had an impact on the game but it was a little bit nervy at the end.
"If we could have got the second goal we would have killed them off but I was delighted with the three points and the clean sheet."
Source: Team Talk
Source: Team Talk