LIP BLOCKING (or "lipping") is the preferred method for melody and blues bends. Lipping is a technique where you cover all holes on the harmonica with your lips EXCEPT the one hole you want to play. You do this WITHOUT using your tongue to block any of the holes. Your pucker should be about the size of a pencil eraser.
Your lips have to be open enough to allow room for one complete hole. If you pucker too much you will lose volume and tone. If you don't pucker enough you will get multiple notes.........This takes practice!!
TONGUE BLOCKING is another technique for playing one note. In tongue blocking you put the harmonica deeper into your mouth than in lip blocking. Your lips are more open and allow 2-3 holes to be open inside the pucker. You use your tongue to cover all but the one hole you want to play. Typically you cover the notes to the left of your mouth and leave the note on the right open....but you can cover which ever side you like. (See Tongue Blocking below to play the 4 hole)
The tongue blocking is also used (in a different form) to cover center holes and leave holes on each end open. This form of tongue blocking is called "tongue splitting" and comes in handy for playing multiple notes and even octaves.
Play around with whichever blocking technique you chose until you can make one note sound good. Then practice moving from one hole to the next. Alternate blow/draw notes along with the mouth movement up and down the harmonica. Play the "C" scale up and down until it becomes smooth and you consistently hit one note and ONE NOTE ONLY.