  The symplectic blob algebra is a physically motivated quotient of the Hecke
algebra $H(\tilde{C}_n)$ with a diagram calculus. We find the blocks for the
symplectic blob algebra for all specialisations of its parameters over the
complex numbers. We determine Gram determinants for the cell modules with
respect to a canonical contravariant form. We show in particular that the
algebra is semisimple over the complex numbers unless at least one of the
"quantisation" parameters, or the sum or difference of two of these parameters
is integral, or the bulk parameter $q$ is a root of unity. We find
decomposition numbers in many of the $q$-generic cases.
