Is "A" inhibiting "B" and vice-versa, a positive feedback loop?
The situation that you presented in which an entity A inhibits the production of another entity B which in turn inhibits A, is a positive feedback.

In a network path or a loop the overall sign of the loop/path is the product of the signs of individual edges (interactions). In this case it is negative times negative which gives a positive sign to the loop. 

In simple words, a positive feedback should lead to an entity promoting its own production, directly or indirectly; it can auto-activate itself, activate an activator or inhibit an inhibitor. In this case A inhibits its inhibitor. Sometimes you can have large loops like this:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 

This is still a positive feedback.

Moreover, your case would exhibit (with the right parameters) the dynamic properties of positive feedbacks (such as bistability and hysteresis). 

Sometimes people also refer to this type of positive feedback as double-negative feedback (which I personally do not like because it is misleading).


  Positive and negative feedback loops may consist of a single component
  that activates and represses directly its own activity, respectively
  (Fig. 1B,C); or they may include several components and involve
  indirect interactions (Fig. 1D–G). The overall sign of a complex
  feedback loop (i.e. positive or negative) depends on the constituting
  elementary interactions (Fig. 1D–G).(1) For example, two mutually
  repressing components form a positive feedback loop (PFL, also termed
  “double-negative feedback loop”).
  
  Mitrophanov &amp; Groisman, 2008 


However, not all positive feedbacks lead to runaway amplification. That is a property of an unstable system. The degradation of the molecules (or death) ensures a finite (and stable) steady state. Sometimes (with certain parameter combinations, as previously mentioned) positive feedbacks can have two stable steady states. Depending on the initial condition, the system can settle to either of the states. In your example the two states can be &mdash; High-A Low-B and Low-A High-B.



Further reading:
Mitrophanov, Alexander Y., and Eduardo A. Groisman. "Positive feedback in cellular control systems." Bioessays 30.6 (2008): 542-555.