if it were bucket/bottle fed I would suspect a gut infection, eg salmonella, and stop milk for a couple of days and give glucose in water instead. Then I would gradually start again with half milk half glucose+water for the next day, providing there was no more blood to be seen. But as you can't catch the calf and it seems ok anyway, then this isn't really an option.
However, if it should start to deteriorate then you will have to catch it somehow and stop it feeding off it's mum until it has recovered. Milk is a perfect feed for bacteria as well as calves so you need to cut it out so the bacteria have nothing to feed on and the calf has a chance to recover.