Mr John Blake is likely to be a fake name. It is not easy to trace the conmen who send these emails, please read my standard answer below for more details. The best thing you can do is ignore these messages and help to ensure that no-one falls for them. That way the money-grabbing conmen will have their source of income cut off.
It is scam. Yahoo (insert any company name here, this answer works for all!) do not run an email lottery, if they did you could guarantee it would be advertised all over their home page. Check out these links and search answers to see how many people 'win' the exact same lottery every day
http://en.wikipedia.org/wiki/lottery_sca...
http://www.hoax-slayer.com/email-lottery-scams.html
Do not click on any links, the page you will be taken to will look authentic but is not
Do not send them a 'courier fee' or any other money
Do not hand over any bank details or passwords
Do report spam
Do delete
Remember there is no such thing as a free lunch. You cannot win a lottery if you haven ot bought a ticket. Anyone can set up an email account with Yahoo, they can use any combination of letters they choose. I could be 'thequeenmother@yahoo.com' or 'disneyfreeticketsgenuine@yahoo.com' That doesn't mean that I am either of these and is fine as long as I don't try to use the address to mislead people. Don't be fooled by legitimate looking email addresses which end the same way as any other free account, don't be fooled by fictitious titles such as 'Dr' or 'Executive Director of Winner Claims'.
(I've answered this question so many times that I now keep this answer on my desktop and just copy and paste....that should give you a clue!)