BTR's or real blacktips?
Well according to Matt's last post - both species.
And although they are both ORV swimming sharks - they have very different swim-glide patterns, and prefer very different habitats. Also BTR and Blacktips tend to have very different temperments. All of which greatly affects their suggested tank sizes. And mean ways the Blacktip & BTR are almost polar oppposites. BTR are native to rocky & coral reefs(as their name suggests) and found in fairly shallow water. Blacktips (C. limbatus) are sharks which seem to prefer deeper open waters. BTR are adapt well to captivity, given enough space. While Blacktips are very difficult to acclimate to captivity - even in a good size pulic aquarium.
Basically for an adult BTR for life - I would suggest something like this as an absolute bare minimum - oval pond - 45 ft long x 18 ft wide x 5-6 ft deep(average depth).
On the other hand an adult Blacktip - would likely need at least a tank -like this
(75ft long x 34 ft wide x 9 ft deep) as a bare minimum. And in truth - it really should be even larger than this.