In singing, blending may refer to (1) bridging registers to eliminate "breaks" in the voice that tend to occur between registers, or (2) blending chest and head resonances to produce a 'mixed' tone that is characteristic of the middle voice.
Techniques to improve blending
Practice using a 5-note chromatic series around the region of the break or transition between registers. When singing the notes and moving between registers, slightly close the last two notes of the lower register when ascending and open when descending. Also, ease off the power before moving to the next register.
Change the vowel sounds. Narrow or "darken" vowels just before the change to a higher register. A technique known as "capping" or "cupping", which involves pulling down the soft palate, can be used to darken the sound.
When moving upward, create more acoustical space, for example by lifting the cheeks as if smiling Use a slight nasality – a "ng" sound.