  There is so far no clear-cut experimental analysis that can determine whether
dipole-dipole interactions enhance or reduce the blocking temperature $T_{B}$
of nanoparticle assemblies. It seems that the samples play a central role in
the problem and therefore, their geometry should most likely be the key factor
in this issue. Yet, in a previous work, J\"onsson and Garcia-Palacios did
investigate theoretically this problem in a weak-interaction limit and without
the presence of an external DC field. Based on symmetry arguments they reached
the conclusion that the variation of the relaxation rate is monotonous. In the
presence of an external magnetic field we show that these arguments may no
longer hold depending on the experimental geometry. Therefore, the aim of this
paper is to evaluate the variation of $T_{B}$ for a model system consisting of
a chain of ferromagnetic nanoparticles coupled with long-range dipolar
interaction with two different geometries. Rather than addressing a
quantitative analysis, we focus on the qualitative variation of $T_{B}$ as a
function of the interparticle distance a and of the external field $h$. The two
following situations are investigated: a linear chain with a longitudinal axial
anisotropy in a longitudinal DC field and a linear chain with a longitudinal
axial anisotropy in a transverse field.
