CELEBRATING FIFTY By Karen Blaker (Contemporary Books: $17.95; 192 pp.). This collection of conversations with women who hover near 50 reads, sadly, like the tepid outtakes of Barbara Raskin's novel, "Hot Flashes." Karen Blaker's subjects suffer from an unfortunate homogeneity both of circumstance and attitude. Most are moms of almost-grown children, or grandmoms; hasn't Blaker noticed that there's about to be a generation of 50-year-old women whose kids are still in elementary school? Most are economically comfortable, educated, naturally reflective women--just the ones for whom turning 50 is likely to be easiest. The book suggests that a woman of 50 needs to be her own best advocate, refusing to accept the received wisdom about what can, and should, be an energetic period in her life. But Blaker does little to empower the reader. Her subjects are too glib, her questions too bland. It's hard not to get angry, reading this effort. Blaker's intended audience deserves more than tea and sympathy.