a spot

The forest was black and silent. A little way into it they reached a fork in the earth path, and Harry, Hermione, and Hagrid took the left path while Malfoy, Neville, and Fang took the right.
They walked in silence, their eyes on the ground. Every now and then a ray of moonlight through the branches above lit a spot of silver-blue blood on the fallen leaves.
Harry saw that Hagrid looked very worried.
"Could a werewolf be killing the unicorns?" Harry asked.
"Not fast enough," said Hagrid. "It's not easy ter catch a unicorn, they're powerful magic creatures. I never knew one ter be hurt before."
They walked past a mossy tree stump. Harry could hear running water; there must be a stream somewhere close by. There were still spots of unicorn blood here and there along the winding path.
"You all right, Hermione?" Hagrid whispered. "Don' worry, it can't've gone far if it's this badly hurt, an' then we'll be able ter -- GET BEHIND THAT TREE!"

I guess, ‘spots’ is better for describing blood. Then why the writer select ‘a spot’ unlike the below ‘spots’?