All of those things that you mentioned that he could do except for the stolen bases are a product of playing in Coors Field for his entire career. He's only hit above 20 homeruns once, last season when he hit 29, he's a career 105 wRC+ hitter, so his OPS in a less extreme park should be noticeably lower as should his batting average, and him being 30 means the days of 30+ stolen bases a season are likely behind him. He's also not a particularly good defender, sure he can play all OF positions but it's more at an average or below level, and with his age can only be expected to decline.
Then there's Stroman who's already a very good pitcher, is much younger and has more control. Simply a much more valuable piece any way that you look at it. Stroman doesn't need to improve one bit from this season and he should still be expected to be easily more valuable than Blackmon over their control years.