  We consider the problem of computing N=2 superconformal block functions. We
argue that the Kazama-Suzuki coset realization of N=2 superconformal algebra in
terms of the affine sl(2) algebra provides relations between N=2 and affine
sl(2) conformal blocks. We show that for N=2 chiral fields the corresponding
sl(2) construction of the conformal blocks is based on the ordinary highest
weight representation. We use an AGT-type correspondence to relate the
four-point sl(2) conformal block with Nekrasov's instanton partition functions
of a four-dimensional N=2 SU(2) gauge theory in the presence of a surface
operator. Since the previous relation proposed by Alday and Tachikawa requires
some special modification of the conformal block function, we revisit this
problem and find direct correspondence for the four-point conformal block. We
thus find an explicit representation for the affine sl(2) four-point conformal
block and hence obtain an explicit combinatorial representation for the N=2
chiral four-point conformal block.
