A Blood Mirror holds the power of a Vampire clan. If it's smashed, every vampire in that clan supposedly turns into its mortal state (which would result in death for old vampires).
As a result of it's importance, the mirrors are guarded by strange statue-esque guardians, and only a member of a clan's bloodline (including any half-fangs) can open the vortex to enter the mirror.
The Dracula Blood Mirror first appears in the episode "Mirror Mirror". Vlad and Boris, with a little help from Eric Van Helsing, read about it's existence in A Painless Cure for Vampires. They enter the crypt, where they overcome the castle's defences and enter the room where the artefact is stored. Ingrid is seen smashing a large mirror. The real mirror is much larger and more impressive.
The mirror appears briefly in "Bad Reflection" when Boris changes into a vampire. It's power is not shown.
In "Sweet Sixteen" Ingrid approaches the mirror for her transformation. Inside the mirror, the vampiric reflection shows Ingrid her 'future'. Suddenly the reflection tears at the glass and Ingrid runs away in fright. The evil reflection is able to free itself and it follows Ingrid all the way to school. Eventually Ingrid and the mysterious entity merge and she becomes a fully fledged vampire.
In "Bad to the Bones" mysterious voices lure Vlad to the Blood Mirror. The guard creatures hurl him in, trapping him in the world beyond. Inside the mirror Vlad encounters not just one evil reflection, but a thousand.
After merging with them, Vlad becomes cruel and wreaks havoc around Garside Grange School. Eventually he is able to separate himself from his reflection. They merge fully back in the mirror room.
Series 4In "Kiss of Death" Vlad orders the guards to open the mirror vortex. The world beyond has changed dramatically since series 3 and there is an area of woodland with a lake. On the other side of the mirror Elizabeta and Malik are under Vlad's control. Vlad 'zones' Elizabeta and throws her into the lake.
Series 5In "Nemesis Rising" Malik attempts to rescue his mother from the lake. He manages to persuade Ingrid to help him. Ingrid opens the vortex and guides Asan and George into the Blood Mirror.
The world on the other side looks the same as in series 4. There are strange winged creatures flying through the trees around them, and large slimy creatures in the lake. Ingrid tells Asan and George to board the wooden boat which is tied at the side of the lake. She warns them not to put their hands in the water (probably so they avoid anything which might have sharp teeth). The sign to which the boat is tied reads 'No Witch Ducking'.
The reason that Malik needed Ingrid's help is that he was unable to open the vortex, one of the first clues that he was not a Dracula, which is confirmed when he visits the other side to learn the truth from his mother.