A collection or gathering of member fleets from one, or more guilds.
Blobs are formed primarily to protect large fleets and critical ship types. By amassing a collection of dozens of fleets in a single location it becomes more difficult for an enemy to attack without immediate overwhelming response and destruction. Parking your fleet on a blob is usually safer than having it sitting atop one of your bases.
Blobs provide all the benefits of a bird flock, fish school, or animal herd - safety in numbers. Blobs give a safe haven for smaller fleets, and allow weaker units like Recyclers and Carriers a place to park unmolested. Players who spend fairly long periods off line find that blobs are the only way to insure that they will not have their bases attacked simply to destroy their fleets.
If a guild has a strong alliance with other guilds the blob may become shared. This increases the size and security of the blob, and gives all the participating guilds a jump off point for mutual attacks in the area. Often members of other guilds, who are parked at the blob, shift to the gate owners guild in order to use the jump gate and join in the mass attack.
Like any herd, blobs attract predators. Having several dozen extra large fleets in one place makes a very juicy target. If an enemy can form a coordinated attack it is possible for a blob to become a killing field. Preventing this requires constant scanning by the owners of guild bases in the region, and active management of the guild members with fleets on the blob.
Usually guilds want to have blobs located on bases with high level Jump gates. There will almost always be a blob formed on a Worm Hole base. Often larger guilds have multiple blobs, usually located in separate Galaxy Clusters. The idea is to make convenient strongholds to amass fleets for coordinated attacks, and to allow those attacks to move out swiftly.