The body of Christ, moreover, is more than just the sum total of believers on earth. While it contains all believers, it does not exist merely because there is a body of believers, but it is a separate entity into which believers are brought — a spiritual union accomplished by the Holy Spirit who creates the believer in Christ, hence as a part of His body. This union is a heavenly existence. The church is not earthly, but heavenly, since its existence is in Christ. The church would exist even if there were no believers, since the church is in Christ, and believers are baptized into a relation to Christ.
Darby does not refer to the assembly as a formal organization. Neither a body of professors nor an external corporation can occupy a relation of identity to Christ. Between Christ and the church as a society there is no organic connection such as exists between the members of a tree and the tree itself. Only individual believers are in Christ, as the branch is in the vine.
There is, in reality, no such thing as Christ dwelling in the church, if the church be viewed as something distinct from the individuals which compose it. If societies may be said to have Christ as their head, it is not by direct union, but mediately; that is, it is because the individuals of which they are composed as in union with Him. The societies may be churches of Christ, but is the individuals who compose them who are members of Christ's body. Only as the assembly is viewed as identical with the actual union of believers can it be said to be the body of Christ. [Clarence B. Bass, Backgrounds to Dispensationalism: Its Historical Genesis and Ecclesiastical Implications (Grand Rapids, MI: Eerdmans, 1960; Grand Rapids, MI: Baker, 1977), 112-3]
I had said that Dispensationalism doesn't really have an ecclesiology. I would have to qualify that further: Dispensationalism does have an ecclesiology, but an ecclesiology without an actual doctrine of the visible church.
According to John Nelson Darby, the founder of Dispensationalism, there is only one church, which is heavenly and objective. This we can see to be roughly equivalent to the 'invisible church.' Believers join this 'invisible church' through faith and purity of life and doctrine, which in Darby's view means joining the assemblies he is in charge of (and no other). There is no real corporate dimension in Darby's ecclesiology with respects to the visible church (only the invisible church), as each individual joins the invisible church as he or she is in relation to Christ. The outward assembly is not considered a "church" per se, except through the mediation of individuals who are members in the invisible church. In other words, there is no actual 'visible church' (according to the Reformed definition of that phrase) in Darby's system. There is only the one invisible church, and individual believers join local assemblies, which partake of the ecclesial status only insofar as their members are all pure in life and doctrine.
Darby of course emphasizes the "local church" or rather the local assembly of believers. He does see that to be essential to the expression of the invisible church. Here is where things get really strange, because it would otherwise seem incomprehensible how one can have such an emphasis on the "local church" yet one does not seem to esteem the visible church. In Reformed ecclesiology at least, the local church IS one particular visible church, and churches coming together in presbyteries and general assemblies constitute the expression of the visible church. Not so in Darby's system, where the two terms, which most people might naturally associate together, are divided. This is why I had initially thought Dispensationalism, at least the classical kind, has no actual ecclesiology, because the whole idea makes no sense to anyone coming at the topic from a Reformed viewpoint, i.e. it is marginally incoherent.
In Reformed ecclesiology, the visible church is expressed in particular local churches. Each congregation comes together to worship God and hear His word, and each of them are indeed churches. This is the visible church. They also are local churches. Thus, at the local level, the two terms "local church" and "visible church" coincide. They refer to institutions God has ordained for His people. To say that there are no direct relations between Christ and a body of believers (as Darby has done) is to deny the very idea of the 'visible church' as the Reformed have defined the term. Darby thus deny the doctrine of the visible church. His idea of the church is truly invisible, "heavenly."
Darby's promotion of local assemblies of believers however might confuse those who read his attack on the assembly of believers having no direct relation to Christ as being a denigration of the visible church. Since in especially Reformed circles, to promote the assembling of believers is to promote the local church which is a visible church, Darby seems to be both affirming and denying the visible church. However, if we look more deeply, we see that Darby dichotomizes between the "local" and the "visible." The "local church" for Darby is a collection of individuals, each of whom should have a relation with Christ. The "visible church" in Reformed theology however denotes the institution consisting of individual members (not a mere collection of individuals), with the institution having a corporate relation with Christ, not just a summary collection of her members' personal relations with Christ. Darby thus denies the "visible church", while affirming the "local church" in his unique sense of the term.
The extreme separatism associated with Dispensationalism and Fundamentalism does not arise because of a desire for doctrinal purity, as it is commonly thought. It arose out of Darby's unique ecclesiology affirming the "local church" while denying the "visible church." Because Darby denies the concept of the visible church, therefore the purity of the invisible church in heaven is brought into consideration at the local church level. Whereas in Reformed ecclesiology, purity is not the goal on this earth just faith and confessional fidelity, in Darby's thought purity is essential in the church. The Eschaton has in a sense dawned among the Plymouth Brethren. This quest for purity drives much of the controversies among the Plymouth Brethren, a quest in futility since Christians will never be perfect this side of heaven, not even in doctrine (although we should strive towards greater godliness in life and doctrine).
So yes, Darby has an ecclesiology, an utterly eccentric one at that. Reading this, now the whole focus on "local church" while denigrating the church in general makes sense, as well as how the doctrine of separation can be taken to the third and fourth degrees.