Why was Mrs. Bloomfield nervous?

Mrs. Blomefield is the vicar's wife. She and her husband have a large family of boys and girls, who often play with Miss Jessie and Flora, Squire Gordon's children. Whenever the children get together, there's always lots of work for the dapple gray pony Merrylegs as they love nothing more than taking it in turns to ride him round the orchard and home paddock.
Merrylegs is a very reliable pony, and doesn't mind giving the children rides. But he doesn't have much time for boys; he tells Black Beauty that they need to be broken in, just like colts. So when he's had enough of them, he throws them off backwards, not out of cruelty, but to teach them a valuable lesson in how to treat horses.
Despite this, Merrylegs has an excellent reputation for being a trustworthy steed, and he's immensely proud of it. Naturally, Mrs. Blomefield is anxious for the safety of her children when they're riding the pony. But Merrylegs' master assures her that they're perfectly safe; Merrylegs will take good care of them. He's a kind, trustworthy animal and his master wouldn't sell him for any amount of money.