The Blasius problem $f'''+ff''=0$, $f(0)=-a$, $f'(0)=b$, $f'(+\infty)={\lambda}$ is exhaustively investigated. In particular, the difficult and scarcely studied case $b <0\leq{\lambda}$ is analyzed in details, in which the shape and the number of solutions is determined. The method is first, to reduce to the Crocco equation $uu''+s=0$, and then to use an associated autonomous planar vector field. The most useful properties of Crocco solutions appear to be related to canard solutions of a slow fast vector field.