Studies have shown that, on the one hand, charge transfer (CT) plays a key role in halogen bonds (D ... X-A) (D = donor; X = halogen atom; A = acceptor), suggesting considerable covalent character of halogen bonding. But, on the other hand, it has been proposed that halogen bonding is dominated by the electrostatic attraction between the electropositive a-hole at the halogen atom X and the electronegative donor D. It has also been well-recognized that the CT from the donor D to the antibonding sigma*(X-A) would weaken and lengthen the X-A bond. Yet, intriguingly, there is a blue-shifting phenomenon in halogen bonding, where the X-A bond contracts with an enhanced stretching vibrational frequency. Here we explored the nature of blueshifting halogen bonds with the iconic case of H3N center dot center dot center dot ClNO2, which exhibits the blue-shifting phenomenon along with a strong CT interaction and its analogous H3N center dot center dot center dot XNY2 (X = Cl, Br, and I; Y = O, S). By decomposing the binding energy to a number of energy components and exploring their energy profiles along with the halogen-bonding distances with the block-localized wave function (BLW) method, we showed that the classical electrostatic interaction is the governing factor for the blue-shifting of the X-N bonds. This is further supported by the similar magnitudes of blue-shifting obtained when NH3 is replaced with atomic point charges in the complexes. Alternatively, by applying external electric field (E-field) along the X-N bond direction, the blue-to-red shifting transition can be identified. This is because both polarization and CT interactions tend to stretch the X-N bond, and both are enhanced simultaneously under the external E-field. Finally, roles of individual energy components are reconfirmed using the force analysis based on the BLW energy decomposition approach.