Generating thin and high density plasma slabs at a high repetition rate is a key issue for ultra-high intensity laser applications, such as plasma photonics, electron acceleration by few laser-cycle pulses, or collisionless shock acceleration of protons to high energies. In this paper, we present a scheme to generate such plasma slabs. It is based on the propagation and collision in a gas jet of two counter-propagating blast waves (BWs). Each BW is launched by a sudden and local heating induced by a nanosecond laser beam that propagates along the side of the jet. The resulting cylindrical BW expands perpendicular to the beam. The shock front, which is bent by the gas jet density gradient, pushes and compresses the plasma toward the jet center. By using two parallel ns laser beams, one on each side of the gas jet, this scheme enables us to tailor independently two opposite sides of the jet, while avoiding the damage risks associated with counterpropagating laser beams. A parametric study is performed using two and three dimensional hydrodynamic (single fluid), as well as kinetic (Fokker–Planck), simulations. This study shows that the BW bending combined with the collision in a stagnation regime increases the density by more than ten times and generates a very thin (down to few micrometers), near to over-critical plasma slab with a high density contrast (>100) and a lifetime of a few hundred picoseconds. Two dimensional particle-in-cell simulations are, then, used to study the influence of the plasma tailoring on proton acceleration by a high-intensity sub-picosecond laser pulse. It is shown that tailoring the plasma, not only at the entrance but also at the exit side of the picosecond-pulse, enhances the proton beam collimation and increases significantly the number of high energy protons, and their maximum energy.