[Answer]-The view blog.views.contacto didn't return an HttpResponse object

Your view doesn’t return an HttpResponse if request.method is not POST.

There is an indentation issue. You need to return an unbound form in case the form is not submitted:

def contacto (request):
    if request.method == 'POST': #Si e formulario es enviado...
        form = Formulario(request.POST)
        if form.is_valid(): #Si son validos los datos del formulario
            return HttpResponseRedirect('/blog/gracias') #redireccion a gracias
    else:  # <-- WATCH WHERE ELSE IS
        form = Formulario() #un Unbound form

    return render(request, 'contacto.html',{'form':form,})

Also see this relevant example that describes this standard form view pattern.