Articulation

Think of vowels and consonants.

When you use a consonant such as T or K or even L, you can use it to "attack" a note - give it a defined start, or to cut it off at the end.

You can also use an attack on the first note in a lick, riff or melodic phrase to help give it mroe definition, and then flow the rest of the notes together.

You can also create a rough texture by using articulation to cut a line up into separate notes. You can flow notes together smoothly by not articulating them and by not emphasizing breath changes when you chagne between inhaled and exhaled breaths.

For any given series of notes, you can choose to articulate (separate notes) or phrase (flow them together). 

Listen to what your favorite players do. Notice when they flow notes together, when they cut them apart, and when they articulate. How does this affect the overall texture of the lick, riff, or melodic line? What would it sound like without those articulations?

You can make vowel sounds most effectively - that is so that your listeners notice them - with your hand cupping technique.

If you start a note with your hands closed and open them while you continue the note you'll get a "Wah" sound. (You often hear a bent note start with a closed cup, with the bend being released as the cup opens - makes a very effective "crying" sound.)

If you start the note with your hands open and then close them you'll hear an "Ooh" sound.

You can make vowels sounds with your tongue as you play, mostly "Eee" and "Ooh." The "Ooh" can sound hollow, while the "Eee" can sound bright, especially if you move the front of your tongue close to the back of your upper front teeth. You can use these effects to create tonal contrasts.

A few additional notes.

When you tongue block, the tip of your tongue is on the harp and thus you lose the ability to do some of the vowel and consonant sounds I describe. However, you gain the ability to create such articulations as the slap and pull, and also to create such textures as rakes, hammers, and shimmers. You can find out more about those techniques here and in my books, Harmnoica For Dummies and Blues Harmonica For Dummies.