Due to their nearly constant absolute magnitudes and old ages, Blue Horizontal Branch (BHB) stars are frequently used as standard candles to study the kinematics and structures of our galaxy. The number of identified BHB stars has significantly increased due to the advent of large scale surveys in the last two decades. Recently, Gaia DR3 was released including a catalog of around 220 million low-resolution spectra (Bp/Rp, or XP hereafter). These data have great potential for identifying many interesting stellar objects including BHB stars. We select BHB stars based on synthetic photometry and the stellar atmosphere parameters inferred from Gaia Bp/Rp spectra. We generate the synthetic SDSS broad-band ugr and Pristine narrow-band CaHK magnitudes from Gaia Bp/Rp data. A photometric selection of BHB candidates is made in the (u-g, g-r) and (u-CaHK, g-r) color-color spaces. A spectroscopic selection in Teff-logg space is applied to remove stars with high surface gravity. The selection function of BHB stars is obtained by using the Gaia DR3 photometry. A non-parametric method, which allows the variation of the vertical flattening q with the Galactic radius, is adopted to explore the density shape of the stellar halo. We construct a full-sky BHB catalog from Gaia Bp/Rp spectra and use it to explore the shape of the inner stellar halo. We present a catalog of 44552 high latitude (|b|>20{deg}) BHB candidates chosen with a well-characterized selection function. The stellar halo traced by these BHB stars is more flattened at smaller radii (q=0.4 at r~8kpc), and becomes nearly spherical at larger radii (q=0.8 at r~25kpc). Assuming a variable flattening and excluding several obvious outliers that might be related to the halo substructures or contaminants, we obtain a smooth and consistent relationship between r and q, and the density profile is best fitted with by a single power law with an index alpha=-4.65+/-0.04.