Tom's imagination is one of his most appealing characteristics, and it's at its best when he's striving to do things as he thinks they should be done. Whenever possible, Tom strives to add ceremony and ritual and all the trappings of making whatever adventure he's involved with seem bigger-than-life.
Therefore, a mere promise between Tom and Huck that they won't give away the secret they share regarding their observations in the graveyard is not weighty enough for the situation. Huck suggests "there orter be writing 'bout a big thing like this. And blood." Tom seizes the suggestion and amplifies it. The oath he pens combines every impression of Robin Hood and pirates and all the other glorious figures he loved to imagine himself to be. The blood oath seals the solemn document, as required by any deep commitment entered into by such heroic and adventurous individuals.