We demonstrate tunable, extremely blueshifted continuum in lambda = 1.055 mu m ultrashort laser pulse filamentation in silica. Close to threshold, the continuum appears as a single, isolated blue peak. The spectral position of the two supercontinuum components can be tuned and a regime with encompassing fundamental and second harmonic is possible to achieve. At higher energies, the continuum expands in bandwidth starting from the blue peak. The spectral dynamics and tunability are explained in terms of X-wave generation and intrafilament pulse splitting which may be controlled by modifying the input pulse focusing conditions.