I will assume:
- v-net-0 was created with something like
ip link add name v-net-0 type bridge
- veth-blue-br was created with something like
ip link add veth-blue-br type veth peer name veth-blue
It is my understanding that the "master" parameter adds your virtual interface "veth-blue-br" into the bridge called "v-net-0".
I find the term "master" to be a bit confusing for bridging, but I suppose from a certain point of view, it makes sense. From the point of view of the "ip" command, which is generic in the sense that it can configure a number of types of interfaces (e.g. bond, vlan - not just bridge), it is useful to have similar terms (e.g. master) when indicating that an interface is dependant upon another. However, if you happen to be familiar with bridging / switching outside of the iproute2 software world (the ip command is part of iproute2), then the term "master" might be a bit confusing.
The obsolete command "brctl", has a sub-command called "addif", and if you read the man page for brctl, there is a reasonable description under "addif" (quoted below), and I believe this is what the "master" parameter in "ip link set" is doing. Note that brctl makes no mention of the word "master". The functionality from brctl has been added to ip/bridge from iproute2. However, there are useful sections from the brctl man page which don't seem to have made it into the ip/bridge man pages.
The command brctl addif <brname> <ifname> will make the interface
<ifname> a port of the bridge <brname>. This means that all frames
received on <ifname> will be processed as if destined for the
bridge. Also, when sending frames on <brname>, <ifname> will be
considered as a potential output interface.