What is claimed is: 1. A temperature measuring device comprising: (a) an elongated high temperature optical fiber, (b) one end of said high temperature fiber having an optically dense film covering the outer surface thereof, thereby defining a region of the covered end of said fiber exhibiting blackbody radiation behavior, said blackbody region and said high temperature fiber able to measure temperatures in the range of 500.degree.-2400.degree. C., said blackbody region having a length to diameter ratio in the range 1/1 to 20/1, (c) said blackbody region emitting radiation primarily in the range of 0.3 .mu.m-1.0 .mu.m within said temperature range of 500.degree.-2400.degree. C., (d) said high temperature fiber having a diameter in the range of 0.025 mm to 2.5 mm and transmitting radiation in said temperature range without substantial absorption thereof, (e) a low temperature fiber having a different diameter than said high temperature fiber and coupled to the other end of said high temperature fiber, (f) a light detector coupled to receive light transmitted by said low temperature fiber and including a narrow-band filter for selecting a narrow optical band wherein relatively small changes in temperature within said band result in relatively large changes in intensity emitted by said blackbody region, said narrow optical band within said 0.3 .mu.m-1.0 .mu.m range, said light detector providing an output signal proportional to the intensity of said received light, and (g) means responsive to said output signal for providing an indication of said temperature measurement. 2. A temperature measuring device as recited in claim 1 wherein said blackbody region has a surface inclined in the range of 20.degree.-70.degree. from the longitudinal axis of said high temperature fiber. 3. A temperature measuring device as recited in claim 1 further comprising a protective layer overlaying said optically dense film. 4. A temperature measuring device as recited in claim 1 wherein said film comprises a noble metal. 5. A temperature measuring device as recited in claim 4 wherein said noble metal is selected from the group of platinum and iridium. 6. A temperature measuring device as recited in claim 5 wherein said high temperature fiber is a sapphire fiber and said device further comprises an alumina protective film overlaying said optically dense film. 7. A temperature measuring device as recited in claim 6 wherein said optically dense film has a thickness on the order of 0.2 .mu.m-10 .mu.m. 8. A temperature measuring device as recited in claim 5 wherein said high temperature fiber is a zirconia fiber and said device further comprises a zirconia protective film overlaying said optically dense film. 9. A temperature measuring device as recited in claim 8 wherein said optically dense film has a thickness on the order of 0.2 .mu.m-10 .mu.m. 10. A temperature measuring device as recited in claim 1 wherein said optically dense film comprises a chemically doped metallic oxide film. 11. A temperature measuring device as recited in claim 10 wherein said chemically doped metallic oxide film is chromium doped single crystal aluminum oxide. 12. A temperature measuring device as recited in claim 1 wherein said low temperature fiber is coupled to said high temperature fiber via an air gap. 13. A temperature measuring device comprising: (a) an elongated high temperature optical fiber, (b) a blackbody region positioned at one end of said high temperature fiber and comprising an optically dense solid oxide material, said blackbody region and said high temperature fiber able to measure temperatures in the range of 500.degree.-2400.degree. C. said blackbody region having a length to diameter ratio in the range 1/1 to 20/1, (c) said blackbody region emitting radation primarily in the range of 0.3 .mu.m-1.0 .mu.m within said temperature range of 500.degree.-2400.degree. C., (d) said high temperature fiber having a diameter in the range of 0.025 mm to 2.5 mm and transmitting radiation in said temperature range without substantial absorption thereof, (e) a low temperature fiber having a different diameter than said high temperature fiber and coupled to the other end of said high temperature fiber, (f) a light detector coupled to receive light transmitted by said low temperature fiber and including a narrow-band filter for selecting a narrow optical band wherein relatively small changes in temperature within said band result in relatively large changes in intensity emitted by said black-body region, said narrow optical band within said 0.3 .mu.m-1.0 .mu.m range, said light detector providing an output signal proportional to the intensity of said received light, and (g) means responsive to said output signal for providing an indication of said temperature measurement. 14. A temperature measuring device as recited in claim 13 wherein said solid oxide material is aluminum oxide doped with chromium. 15. A temperature measuring device as recited in claim 14 wherein said chromium is doped in the amount between 0.1-20% by weight. 16. A temperature measuring device as recited in claim 14 wherein said high temperature fiber is a sapphire fiber and said device further comprises an alumina protective film overlaying said optically dense aluminum oxide doped with chromium. 17. A temperature measuring device as recited in claim 14 wherein said high temperature fiber is a zirconia fiber and said device further comprises a zirconia protective film overlaying said optically dense aluminum oxide doped with chromium.