What your asking is basicly a paradox, you cannot have both at the same time.
A block string is an air-tight string of moves that leaves no gaps in the string to make it safe against any attempt to punish.
A frame trap is a intentional gap in a block string to bait a normal to come out but the setup is weak to reversal attacks that goes through normals like Ryu’s mp.SRK
If your looking for a beginner friendly frame trap then something like this is quite easy to learn (but dosnt net alot of damage) and very safe if you mess up any timings.
c.LP, s.MP, c.MK xx EX Hadoken (there isnt any timing to this string, just hit the moves one after the other as if you were comboing them)
The frame trap happens between c.LP & s.MP and since c.LP is +2 on block and s.MP has 5 startup frames.