This will search through your Simulink model for any constant containing the value of '8'. However, when you retrieve the 'Value' field of the Constant it is stored as a string, so it will only find constant that have the value of 8, not constant blocks that reference a variable holding the numeric value of 8.
nModelName = 'YourModel';
load_system(nModelName)
sys = get_param(nModelName, 'Handle');
gb = find_system(sys, 'BlockType', 'Constant');
h = find_system(gb,'Value','8');
if ~isempty(h)
for a = 1:length(h)
disp([get_param(h(a), 'Parent'), '/', get_param(h(a), 'Name')])
end
end
