The transition from the thulium:1G4level into the ground state is a candidate for the construction of a IR pumped blue upconversion laser. We report on the excitation of the Tm:1G4level in the Yb:Tm doped crystals GdAlO3, LiGd(MoO4)2, Y3Sc2Ga3O12and Gd3Ga5O12. The fluorescence spectra are measured in a range from 300 to 2300nm. The lifetimes of the Tm:1G4, Tm:3H4and Yb:2F5/2levels are determined as a function of the dopant concentrations. Furthermore, energy transfer from thulium to terbium is discussed. The results are evaluated in view of the construction of an efficient blue thulium upconversion laser. From the investigated crystals, Y3Sc2Ga3O12proves to be the most suitable host. Comparison of the measured data with values from the literature suggests that Y3Sc2Ga3O12is an even more promising crystal than the low phonon host LiYF4. It is shown that the optimum thulium concentration is of the order of 0.1at% for a ytterbium concentration of 10at%.