Your blood cannot be significantly acidic or alkaline relative to normal. The process of homeostasis keeps all of your blood and fluids within most cells within a very specific pH range - otherwise it's a complete disaster. If you had truly acidic blood, you'd be dead.
Also, lemon is not alkaline. Lemon juice has a pH of between 2-3, varying with individual fruits due to citric acid content. Regardless of the pH of lemon juice, all food that's digested is first processed with hydrochloric acid (naturally produced in the stomach, which is a pH of 1 - 2), and then buffered to an appropriate pH for digestion in the rest of the digestive tract.
Anyone who claims blood acidity has anything to do with anything except, say, renal failure and imminent death is quite frankly being ridiculous.
http://www.quackwatch.com/01Quackery...SH/coral2.html