Did a little more poking around and found this (copy pasta from another site)
How do THC and CBD interact with CB1 receptors?
THC fits into CB1 receptors like a key in a lock. Scientifically, this phenomenon is referred to as orthosteric binding, which causes CB1 receptors to initiate a cell signalling cascade that inhibits the release of other neuro-transmitters. CBD, too, has an affinity for CB1 receptors, but binds to a different site in what is referred to as allosteric binding. Unlike THC, instead of initiating a signalling cascade, CBD modifies the shape of the receptor, altering its response to THC and other endocannabinoids. Allosteric modulators can either increase or reduce a receptor's signalling capacity, so if THC can be compared to a key and the receptor to a lock, CBD would either widen or narrow the lock, making the door easier or harder to open.
Positive modulators increase the signalling capacity of receptors, while negative modulators have the opposite effect.
In conclusion...
Further research is needed to fully understand the way CBD interacts with THC. Currently, CBD is defined by science as a negative allosteric modulator because it reduces activation of CB1 receptors, so in this sense it should be safe to say CBD "blocks" the effect of THC is some way. However, studies in rodents suggest this phenomenon is dose dependent, meaning CBD could either reduce or stimulate the effects of THC depending on how much or how little of it you take.
At present, it's hard to tell the effect a cannabinoid will have on the body before you actually take it. The dose, the THC:CBD ratio and the patient's metabolism are all factors that influence how the body reacts to these substances. Unluckily, the only way right now to modulate the effect CBD has on the body is to start on a low dose and gradually increase it until the desired effect is reached.